Add support for Ecliptic coordinates in wcs_to_celestial_frame and celestial_frame_to_wcs

[astrofrog]

The functions wcs_to_celestial_frame and celestial_frame_to_wcs in astropy.wcs.utils don't currently support the ecliptic frames from astropy.coordinates, but this should be added. See the following paper for the conventions, in particular there are mentions of ecliptic and helioecliptic frames:

http://www.atnf.csiro.au/people/mcalabre/WCS/ccs.pdf

It looks like RADESYS should be used for ecliptic coordinates, so the bottom line is that someone needs to carefully read through that paper and implement things correctly (a nice little self-contained project!)

[eteq]

That's very subtle. I think the idea here is that in principal the FK4/FK5/ICRS reference systems can each separately be combined with a particular specification of the ecliptic coordinates at a certain equinox. That is, FK4/FK5/ICRS technically all have slightly different orientations even if they're all at the same equinox. I believe our ecliptic coordinates are essentially those where RADESYS is ICRS. But honestly I'm not sure if real data makes any distinction between these (at the same equinox).

So more reading is still needed... But also I suspect it would help to get a few files that actually have ELON/ELAT WCS and try to see if any differences can be detected. (I've never seen such a thing in the wild myself, though...)

[Jurajhh]

Hello, I would like to open a PR :-)

[pllim]

@Jurajhh , thank you for your interest. Please refer to http://astropy.readthedocs.io/en/latest/development/workflow/development_workflow.html for contributing guide.

[Jurajhh]

I apologize @pllim - my intention was to collaborate within the group I would like to apply for within GSOC '18 and after some communication with Brigitta Sipocz about my prefered projects, she suggested me to open a PR in the stingray group.

Anyway, I wish you good luck, I might return here, but currently I would like to delve into the topics surrounding the Phase-resolved oscillations project which I now consider myself to be most suitable for.