* the time operation aggregation was broken

routine yac_cget_model_timestep_id now aborts in case it is called for a field that is neither SOURCE or TARGET

adds const to field_name argument in interface routine yac_cget_role_of_field_by_name and yac_cget_role_of_field_by_name_instance

adds testing of routines yac_get_start_datetime, yac_get_start_datetime_instance, yac_get_end_datetime, and yac_get_end_datetime_instance

* contrib/read_fortran_data.c
*contrib/read_icon_data.c