Merged requested to merge kaluza/mhm:datetime into develop
Hosts year, month, day, hour.
Also has prev_year and so on private. These are needed to get is_new_year,...
Everything is updated on increment. With this lots of code is not distributed and doubled over mrm_eval, mhm_eval and mrm_write_fluxes.
datetimeinfo also contains iLAI and yID since these integers are fully derived by datetimeinfo and used often.