rename offset_to to offset_byand have it use units

[eteq]

This idea was triggered by looking at the implementation of another astrowidgets-conforming API in spacetelescope/jdaviz#705 :

1. I think the offset_to method is better named offset_by. This aligns better with astropy.coordinates and I think it makes it just a bit clearer that you are giving the delta coordinates instead of actual coordinates
2. I think we can dispense with the skycoord_offset keyword by instead using the units of dx/dy - if they are either pixel or dimensionless that can be interpreted as pixel offsets, and any other unit can assume SkyCoord/other WCS ("other wcs" can be implemented in the future, but I'm just trying not to accidentally specify ourselves out of it)

My proposal would be to keep in offset_to with its current API but have it start raising a deprecation warning (and internally it would call the new offset_by)

And yes, offset_to was my original suggestion. But I think it was wrong after seeing it in action!

[eteq]

Oh, and @pllim saw this idea already in spacetelescope/jdaviz#705, but @mwcraig do you have any thoughts pro- or anti- this?

[mwcraig]

I'm fine with this -- can it wait until we make some progress on the refactoring that @pllim started or does this need an implementation ASAP?

[pllim]

I don't foresee being able to refactor anything over here in the near future. Also see #131 (comment)

[pllim]

I see two different things proposed here. I think it is more practical to keep offset_to and offset_by separate; i.e:

• offset_to: Deprecate. Keep old API.
• offset_by: New API only.

(Maybe this was what you already proposed above... 🤷 )