CI update and automated checking

With this Merge Request a couple of changes in the CI/Checking workflow are proposed:

• the CI-run divides into 4 stages now:
  1. Info about ENV-VARS
  2. Building of multiple executables (GCC73, GCC83, Intel18, with/without OpenMP support, with/without MPI, always release + debug) -> 18 artifacts
  3. Valgrind test: The GCC73, GCC83 and Intel18 serial-debug artifacts run the example domain to check for memory consumption/leaks
  4. Check cases: All artifacts from stage 2 are used to run all mHM check cases
• a new checking suite run_mhm_checks.py is provided to run the mHM check cases with a given mHM executable (see stage 4)
  • a description can be found in checks/README.md
  • the old check_mhm_cases is not working anymore since it depends on the old module system (it may works, if you load chspython and cdo by hand)
• the moduleLoadScripts folder has been updated to work with the new easybuild toolchains on EVE

Problems

• The MPI checks revealed, that case_05 and case_07 fail, when mHM is ran by MPI. In case of Intel + debug + MPI, also the case_04 fails. @kaluza : Is that a problem?
• one CI run now lasts for 25 min. @thober Is that a problem?