Refactor: Enable pFUnit-tests and refactor cmake workflow

This merge refactors the whole cmake workflow:

• create cmake modules:
  • checkfortranpreprocessor.cmake: check for -fpp or -cpp and add function cpp_definitions
  • compileoptions.cmake: add options for MPI, OpenMP, Lapack and Coverage
  • version.cmake: read version and date from files version.txt and version_date.txt
• shrink CMakeLists.txt to only compile the executable from mhm_driver.f90
• add CMakeLists.txt in src/ to compile mhm_lib from all mo_*.f90 files
  • refactor compiler flags (don't alter CMAKE_Fortran_FLAGS globally)
  • safer setting of included and linked paths
• add tests/ folder in src/ containing pfUnit tests
• added message about usage of MPI and OpenMP in mhm_driver.f90
• Refactored Cmake options to have correct type