Prepare KATpoint so it could be made an Astropy affiliated package in PI#6

The goal of this ticket is to be able to make the core of KATpoint an Astropy affiliated package in PI#6. To do this it will need to have the MeerKAT specific areas removed to a separate adaptor package (which I propose should retain the KATpoint name) and a telescope independent interface created to use Astropy objects (which will probably have a telescope independent name - suggestions welcome). However, it will not have to support all types of telescope - purely be clean enough that others might be able to do that in the future.

The core of the work is identified by D.Terrett in SIM-268. In a discussion with Dave and l.schwardt we believe all of these could be achieved this PI. There were also some other work identified which included:

• Verifying the changes in the test routines brought about by the use of Astropy is in line with MeerKAT expectations.
• Replacing the bulk of the KATpoint projection.py with the Astropy WCS set of routines. However, KATpoint has two non-WCS projections (CAR and SSN) which are not supported by WCS lib. Also Astropy WCS is somehow tied to FITS and I am not sure if this is relevant.
• Refactoring delays and objects strings so that MeerKAT specific  aspects are removed to a MeerKAT specific interface.
• Removing any trace of the Xephem catalogue format.
• Deciding how to represent satellite objects (I must admit I didn't write down exactly what happened here. I think there was a way forward, but Dave and Ludwig may need to discuss).
• There was also associated work that Ludwig is doing on delays to support NCRA.