GNSS-SDR v0.0.13 released

This is a maintenance release which includes bug fixes, compatibility with the most recent compiler versions, some code optimization and the addition of minor new features. Most relevant changes with respect to the former release are listed below:

Improvements in Efficiency:

• Faster internal handling of Gnss_Synchro objects by reducing the amount of copying via adding noexcept move constructor and move assignment operators, so the move semantics are also used in STL containers.
• All std::endl have been replaced by the '\n' character, since there is no need to always flush the stream.
• Performed a stack reordering of class members that seems to offer statistically better performance in some processor architectures and/or compilers.
• Add building option ENABLE_STRIP to generate stripped binaries (that is, without debugging symbols), smaller in size and potentially providing better performance than non-stripped counterparts. Only for GCC in Release build mode. Set to OFF by default.

Improvements in Maintainability:

• Improved usage of smart pointers to better express ownership of resources.
• Add definition of std::make_unique for buildings with C++11, and make use of it along the source code.
• Private members in headers have been sorted by type and size, minimizing padding space in the stack and making the files more readable for humans.
• Simpler, less error-prone design of the GNSSBlockFactory class public API and internal implementation.
• Simpler API for the Pvt_Solution class.
• Improved system constant definition headers, numerical values are only written once.
• Improved const correctness.
• The software can now be built against the GNU Radio 3.9 API that uses standard library’s smart pointers instead of Boost’s. Minimum GNU Radio required version still remains at 3.7.3.
• The software can now be built against Boost <= 1.73 (minimum version: 1.53).
• Fixed building with GCC 10 (gcc-10 and above flipped a default from -fcommon to -fno-common, causing an error due to multiple defined lambda functions).
• Fixed warnings risen by GCC 10 and Clang 10.
• Various improvements in the CMake scripts: better decision on the C++ standard to use; simplifications for various API dependency and environment versions requirements, with more intuitive naming for variables; fixed the ENABLE_CLANG_TIDY option; better GFORTRAN module; and broader adoption of the modern per-target approach.

Improvements in Portability:

• The software can now be cross-compiled on Petalinux environments.
• Removed python six module as a dependency if using Python 3.x.
• Make use of std::span if the compiler supports it, and use gsl-lite as a fallback. The latter has been updated to version 0.37.0.
• Improved finding of libgfortran in openSUSE and Fedora distributions.
• Improved interface for FPGA off-loading.
• Allow a random name for the build type. If not recognized, it is set to None. This allows packaging in some distributions that pass an arbitrary name as the build type (e.g., Gentoo) to avoid unexpected compiler flags. The building option ENABLE_PACKAGING must be set to ON when packaging.
• Do not stop the receiver if SysV message queues cannot be created.

Improvements in Reliability:

• Fixed a bug in GLONASS GNAV CRC computation.
• Fixed a bug in GLONASS time year.
• Fixed a possible buffer overflow in the generation of RTCM messages.
• Fixed bugs which could cause a random crash on receiver stopping.

Improvements in Reproducibility:

• Improved reproducibility of the volk_gnsssdr library: Drop compile-time CPU detection.

Improvements in Testability:

• Add building option ENABLE_BENCHMARKS, which activates the building of benchmarks for some code snippets, making it easier to developers to benchmark different implementations for the same purpose. Set to OFF by default.

Improvements in Usability:

• Do not pollute the source directory if the software is built from an out-of-source-tree directory. Downloaded external sources and test raw files are now stored in a ./thirdparty folder under the building directory. In case of an out-of-source-tree build, the generated binaries are stored in an ./install folder, also under the building directory. The old behavior for generated binaries is maintained if the building is done from any source tree subfolder (for instance, gnss-sdr/build): in that case, binaries are stored in the source tree (under gnss-sdr/install).
• Defined new GNSS-SDR.GPS_banned_prns, GNSS-SDR.Galileo_banned_prns, GNSS-SDR.Glonass_banned_prns and GNSS-SDR.Beidou_banned_prns configuration parameters. The user can specify lists of satellites that will not be processed (e.g., GNSS-SDR.Galileo_banned_prns=14,18 since Galileo E14 and E18 satellites are not usable for PVT). Satellites on those lists will never be assigned to a processing channel.
• Added a Matlab script to quantize the input signal with a given number of bits per sample.
• Fixed the building option -DENABLE_LOG=OFF, which strips internal logging from the binary and can help to reduce its size and ultimately to speed up the receiver. In binaries with enabled logging, it still can be disabled by passing the command line flag --minloglevel=3 to gnss-sdr. This can be relevant in embedded devices with scarce storage capabilities.
• Fixed a bug in the Signal Sources configuration that made the number of samples parameter ignored when too large (that is, when set larger than 2^31-1). Now the samples parameter accepts values up to 2^64-1, that is, 18,446,744,073,709,551,615 samples.
• Fixed a bug in the forwarding of NMEA messages to a serial port (configuration of the PVT.nmea_dump_devname parameter was ignored).
• Updated version of the Contributor Covenant to version 2.0. Added badge in the README.md file.

As usual, compressed tarballs are available from GitHub and Sourceforge.

In order to make GNSS-SDR more easily referenced, and to promote reproducible research, each software release gets a Digital Object Identifier provided by Zenodo. The DOI for GNSS-SDR v0.0.13 is 10.5281/zenodo.3965566.