adds weights to interface of routines yac_interp_grid_relocate_src_tgt_pairs and yac_dist_grid_pair_relocate_point_pairs

improves error checking in interp_grid routines yac_interp_grid_get_src_coordinates and yac_interp_grid_get_tgt_coordinates

* thanks to Uwe Schulzweida for spotting this

adds new interp_grid routines yac_interp_grid_do_cell_search_src and yac_interp_grid_do_cell_search_tgt

interp_weights routine yac_interp_weights_add_wsum and yac_interp_weights_add_sum will now automatically choose a better weight option if possible

* link for writing to file were not correctly generated for direct_weights(one to one mapping)