Fix ned atpy

The readthedocs build is currently failing because there is a non-optional dependency on ATpy in astroquery.ned. However, there shouldn't even be an ATpy dependency in the first place since astropy.table can deal with VO tables, so this PR fixes this. I noticed in passing that there is a lot of duplicated code in astroquery.ned, so it might be worth having a ``NED`` class that has methods for the various queries, and share methods for overlapping functionality (such as converting to an astropy.table Table).

There were also a number of PEP8 issues with this file that are now fixed.