Switched from ATpy to Astropy for tables

This changes the ``tabular_data`` factory to use Astropy instead of ATpy.

@ChrisBeaumont - I can't remember whether we had decided to do this, or to provide a backward-compatible hook for ATpy?

Also, ``TabularData`` still uses ATpy, but it's only used for a couple of examples, so should those just be updated to use the ``tabular_data`` factory, and ``TabularData`` be removed? (it doesn't seem to be used anymore?).