New version of the GCC C compiler installed

The GCC C compiler version 7.3.1 has been installed and will be available with module load gcc. As before, the versions 4.8.5, 6.3.0 and 7.3.0 are also available, specifically 4.8.5 without loading any modules, 6.3.0 with module load gcc/6.3.0 and 7.3.0 with module load gcc/7.3.0.