Merged requested to merge muellese/mhm:mrm_standalone_remove into develop
This MR removes the pre-processor directive
MRM2MHM. So mHM and mRM are coupled by default.
In addition the standalone version of mRM is removed from the code-base.
For people in need, there is still the (unmaintained) repo for mRM: https://git.ufz.de/mhm/mrm
Fixes: #147 (closed), #103 (closed)