Replace healpy with astropy-healpix and add new API to directly support SkyCoord objects

[MetinSa]

This pull request implements the suggestions in #19, #20 and #23.

In addition, it will lead to a breaking change in the existing ZodiPy API. Although a new SkyCoord api get_emission_skycoord is introduced, the old functions like get_emission_pix and get_emission_ang still remain which support more healpy-like conventions in terms of the function arguments.

[MetinSa]

TODO: update usage docs page to use SkyCoords API for most usecases

[MetinSa]

Hi, @lpsinger! I am currently working on some of your suggestions from the pyOpenSci review and have a question regarding the frames used in the SkyCoord object for rotation purposes.

Conversion between coordinate frames is done using healpy.Rotator but could be done more simply and idiomatically with SkyCoord frame transformations.

Do you know which specific coordinate frame implementations would best correspond to the ["E", "G", "C"] used inhealpy.Rotator? I find that I am able to transform between BarycentricMeanEcliptic, Galactic and ICRS frames trivially, and it seems to reproduce the transformations I had before with healpy. The HeliocentricMeanEcliptic frame seems to be more difficult to use in the context of user provided pointing.

Perhaps you have some insight here too @pllim?

[pllim]

Re: Coordinate frames -- I am not an expert but perhaps @ayshih can advise or ping someone who can? 🙏