run_merge_mapping_files.sh should add grid dimensions to the output file

The maps output by the runoff_map tool (in tools/mapping/gen_mapping_files/runoff_to_ocn) puts the source and destination grid dimensions on the output file, in netcdf dimensions ni_a, nj_a, ni_b and nj_b. However, the ncl script run by run_merge_mapping_files.sh does NOT copy these dimensions to the final output file. It should.

This is not a problem as far as the coupled system is concerned, but these dimensions are needed if you want to run the check_map tool on the resulting mapping files.