Introduce a simple way to specify WCSGeometry

[cdeil]

The current method we have to specify WCSGeometry is pretty complicated:

• https://hips.readthedocs.io/en/latest/getting_started.html#make-a-sky-image
• https://hips.readthedocs.io/en/latest/api/hips.WCSGeometry.html

from astropy.coordinates import SkyCoord
from hips.utils import WCSGeometry
geometry = WCSGeometry.create(
     skydir=SkyCoord(0, 0, unit='deg', frame='galactic'),
     width=2000, height=1000, fov="3 deg",
     coordsys='galactic', projection='AIT',
)

---

In contrast, Aladin Lite and ipyaladin have this nice simple way:
https://github.com/cds-astro/ipyaladin/blob/master/examples/2_Base_Commands.ipynb

---

The main difference is that they have target which can be a string like "Crab nebula" that's looked up via SESAME, or it can be a string representation of the sky coordinate like "0. 0.", whereas we currently have to import SkyCoord and create it, which means two extra lines.

There is
http://docs.astropy.org/en/stable/api/astropy.coordinates.SkyCoord.html#astropy.coordinates.SkyCoord.from_name to look up from SESAME, I don't know if they have a Skycoord string parsing functionality???

So I think we should also access skydir strings (maybe rename to target) and the simple interface.

In addition, we might want to allow the geometry parameter in make_sky_image to be a dict, and if it is, call WCSGeometry.create_from_dict() for the user as a convenience. I.e. this would work:

geometry = dict(target='crab', fov='10 deg', frame='galactic')
make_sky_image(geometry, hips_survey)

[cdeil]

I see that this issue was partly implemented by @adl1995 in #100.

The main missing pieces are to

• use this simpler from dict method in the high-level docs getting started example (and plot_*.py files in docs)
• make it possible / easier for someone looking at the make_sky_image docstring to figure out what options they can pass, i.e. link from there to the other docstring of the method that is called.

[cdeil]

@tboch - I think in terms of convenience API to define common WCS in a simple way, we might want to just re-use the one from https://github.com/cds-astro/ipyaladin and get rid of our WCSGeometry, or at least add a classmethod to suport what ipyaladin does?

Would be nice if those two Python HiPS packages had a similar API, no?

Users would always be able to make any astropy.wcs.WCS and use that, so really the full functionality remains. This is just for the simple end-user sugar API that most people will use.