Checking for transformability of SkyCoords could handle 2D shapes better #5890

from astropy.coordinates import EarthLocation, SkyCoord, get_moon
import astropy.units as u
from astropy.time import Time
import numpy as np

greenwich = EarthLocation.of_site('greenwich')
times = Time("2017-03-20T12:00:00") + np.linspace(-2,2,3)*u.hour
moon = get_moon(times, location=greenwich)
targets = SkyCoord([350.7*u.deg, 260.7*u.deg], [18.4*u.deg, 22.4*u.deg])
targets.transform_to(moon[:,np.newaxis])

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-10-327087abec51> in <module>()
----> 1 targets.transform_to(moon[:,np.newaxis])

/Users/sl/anaconda3/lib/python3.6/site-packages/astropy/coordinates/sky_coordinate.py in transform_to(self, frame, merge_attributes)
    452         # We could remove overlaps here, but the init code is set up to accept
    453         # overlaps as long as the values are identical (which they must be).
--> 454         return self.__class__(new_coord, **frame_kwargs)
    455 
    456     def __getattr__(self, attr):

/Users/sl/anaconda3/lib/python3.6/site-packages/astropy/coordinates/sky_coordinate.py in __init__(self, *args, **kwargs)
    210         args = list(args)  # Make it mutable
    211         copy = kwargs.pop('copy', True)
--> 212         kwargs = self._parse_inputs(args, kwargs)
    213 
    214         # Set internal versions of object state attributes

/Users/sl/anaconda3/lib/python3.6/site-packages/astropy/coordinates/sky_coordinate.py in _parse_inputs(self, args, kwargs)
    358                     raise ValueError("Coordinate attribute '{0}'={1!r} conflicts with "
    359                                      "keyword argument '{0}'={2!r}"
--> 360                                      .format(attr, coord_value, valid_kwargs[attr]))
    361                 valid_kwargs[attr] = coord_value
    362 

ValueError: Coordinate attribute 'obsgeoloc'=<CartesianRepresentation (x, y, z) in m
    [[( 3380626.24081659, -2114946.52919111,  4961181.87025111),
      ( 3380626.24081659, -2114946.52919111,  4961181.87025111)],
     [( 3986386.39265932,  -139720.62235739,  4960258.04040856),
      ( 3986386.39265932,  -139720.62235739,  4960258.04040856)],
     [( 3520492.65007889,  1873089.76476866,  4961111.20464583),
      ( 3520492.65007889,  1873089.76476866,  4961111.20464583)]]> conflicts with keyword argument 'obsgeoloc'=<CartesianRepresentation (x, y, z) in m
    [( 3380626.24081659, -2114946.52919111,  4961181.87025111),
     ( 3986386.39265932,  -139720.62235739,  4960258.04040856),
     ( 3520492.65007889,  1873089.76476866,  4961111.20464583)]>