Add method in NestedHybridGrid to write the nnc table to file #469

The nnc_table is a pandas.DataFrame which is stored as a property on the NestedHybridGrid class.

Users need to save the nnc_table to disk, as it will be required again after property upscaling to generate the final NNC table for the simulator.

Currently they need to do:

nhg = NestedHybridGrid(..)
nhg.nnc_table.to_csv(filename, index=False)

it would be nice with a convenience method that wraps the above e.g.

nhg = NestedHybridGrid(..)
nhg.write_nnc_table(filename)