add single run namelists for each test domain

Merged Sebastian Müller requested to merge muellese/mhm:add_test_domain_namelists into develop

This MR adds minimal namelists to each test domain, so users are able run the test domains separately.

mhm_outputs.nml, mrm_outputs.nml and mhm_parameters.nml are softlinks to the top level files to keep maintenance low.

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