Simplify PosTools

[AlecThomson]

Hi @maaijke,

There seem to be a few lurking errors in the GitHub here related to casacore and the troubles it can cause with positional info. Further, there is a quite a bulk of code to maintain in the related PosTools.py. In there there is a switch as well to use either casacore or ephem.

Looking at the code, I think that a bunch could be replaced with calls to the appropriate astropy.coordinates and the appropriate frames. What do you think?

[maaijke]

Also here, this is something I have been wanting to implement for quite some time now. It would be good to become completely independent of both pyephem and casacore (apart from the MS interface)

[AlecThomson]

Glad to see that RMextract is being 'modernized' :) So you might want to replace PyEphem interface with SkyField as PyEphem is no longer being supported. Although super-accurate positions are not all that critical for ionosphere modeling.

@twillis449 is there any reason this can't be done with vanilla astropy?

[twillis449]

You are quite correct - astropy should be able to do anything SkyField does.