BUG: Frame attributes in sky coordinates do not obey Numpy broadcasting rules

Originally prompted by @mhvk in https://github.com/astropy/astropy/pull/8805#discussion_r291744415.

The crux of the issue is this: this doesn’t work:

from astropy import units as u
from astropy.coordinates import FK5
from astropy.time import Time

FK5(0*u.deg, 0*u.deg, equinox=time.Time(['J2000', 'J2001']))

whereas this does:

FK5([0]*2*u.deg, [0]*2*u.deg, equinox=Time(['J2000', 'J2001']))

The latter proves that there’s nothing in the machinery to prevent this all working the way you’d want, but the fact that the former fails is a sign that the broadcasting is not working the way you’d expect - e.g. 1 + np.random.randn(2) works just fine so it seems like the former example here should.

I consider this a bug because it was always the intent for broadcasting to work naturally in places like this.

cc @adrn

[lpsinger]

A question that arises is what to do with setting an attribute after the fact. It may not be crazy to insist that for that case the attribute must still be broadcastable to the instance’s shape, so that we can guarantee that the instances shape does not change.

I agree with that.

[mhvk]

See also discussion following https://github.com/astropy/astropy/pull/13398#discussion_r929752132, which showed that even the person who wrote the initial code continues to think that broadcasting works, when it at present does not. EDIT: see in particular some failures described in https://github.com/astropy/astropy/pull/13398#discussion_r931151751