* suggested by Nils-Arne Dreier

replaces interface routine yac_cget_pair_rootcomm with yac_cget_comps_comm (yac_fget_pair_rootcomm with yac_fget_comps_comm)

moves the registering of fields in the coupling configuration from routine yac_cdef_field_mask to routine yac_instance_add_field

* it is not required to provide a configuration file anymore

makes use of couple_config routine yac_couple_config_field_is_valid in routine yac_cget_nbr_fields_instance and yac_cget_field_names_instance

routine get_field_configuration now only generates information for field configuration for which both source and target field are defined