fix LAI selection

Merged Sebastian Müller requested to merge fix_LAI_selection into mpr_finalize
  • added nLandCoverPeriods, nlaiPeriods to namelists
  • moved yId, iLai counter into own class aggregateperiod
  • moved all land cover and lai related stuff to mo_common_datetime_type.f90
  • updated the mo_mhm_mpr_interface.f90 routines
  • yet TODO:
    • refactor all changed variables
    • update restart routines

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