Adding OpenBLAS, setting default for GNU compiler, sourcing mpivars + Update gcc,g++,gfortran and OpenMPI

This MR is introducing the following changes:

• adding OpenBLAS to have a BLAS and LAPACK library available,
• setting the default {gcc,g++,gfortran} to the latest version with update-alternatives,
• adding source mpivars.sh to /etc/bash.bashrc to extend $PATH and $LD_LIBRARY_PATH in order to make the MPI wrappers usable directly,
• setting an default entry-point for the container (to compensate shell not found error seen with the gitlab runner),
• Removing the OS Repo for tumbleweed. The Repo for leap is already included in the base image. Mixing tumbleweed and leap leads to incompatibilities, e.g. MPI was not usable due to incompatibilities in the UCP library.

In addition this MR:

• replaces {gcc,g++,gfortran}-{7,8} with {gcc,g++,gfortran}-9,
• updates OpenMPI3 to OpenMPI4.

If needed we could keep the older versions in the image, but this would lead to a large size.