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

* this is more a theoretical issue with the C Standard
* in practice this should make no difference

adds check for invalid timesteps to couple_config routine yac_couple_config_get_source_timestep and yac_couple_config_get_target_timestep