transform_to(AltAz) de-scalarizes if input has a distance #3920

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