transform_to(AltAz) de-scalarizes if input has a distance

[eteq]

Surprising behavior noted (indirectly) by @bmorris3 :

>>> aa = AltAz(location=EarthLocation.from_geodetic(0*u.deg, 0*u.deg, 0),obstime=Time('2015-1-1'))
>>> m=SkyCoord(10*u.deg,3*u.deg)
>>> m.transform_to(aa).shape
()

That part makes sense: the input is a scalar so the output is too

>>> aa = AltAz(location=EarthLocation.from_geodetic(0*u.deg, 0*u.deg, 0),obstime=Time('2015-1-1'))
>>> m=SkyCoord(10*u.deg,3*u.deg, 1*u.AU)
>>> m.transform_to(aa).shape
(1,)

oops! For some reason it decided to switch to a length-1 array coordinate. Definitely should still be a scalar.

This is definitely a bug and not at all intended, but changing it is breaking backwards-compatibility. How do we want to address this? Deprecation? Or leave it in for 1.0.x but change it for 1.1.x?

I'm not sure how used this functionality is right now, so it may not be an issue to slip in the change...

cc @adrn @astrofrog

[astrofrog]

I don't think we really need to preserve backward compatibility for buggy behavior like this. But can you give an example of realistic code that would break because of this assumption to see what we can do?