assign_wcs should be able to evaluate the WCS with non-coordinate inputs

[stscijgbot]

Issue JP-930 was created by Nadia Dencheva:

It will be useful to have a way to pass non-coordinate inputs to the WCS object and evaluate the transforms using those inputs. For example, the Nirspec WCS pipeline now is a compound object which includes the transforms for all slits/shutters/slices. Evaluating it is a two step process:

 
{code:java}
im = ImageModel(input_file)
slit_A200_1_wcs = nirspec.nrs_wcs_set_input(im, 'S200A1')
ra, dec, lam = slit_A200_1_wcs(x, y)
{code}
Instead it will be much more convenient to do

 
{code:java}
im = ImageModel(input_file)
ra, dec, lam = im.meta.wcs(x, y, slit_name='S200A1')
{code}
This requires changes in gwcs and  astropy.modeling. In GWCS the work is tracked in [spacetelescope/gwcs#152|https://github.com/spacetelescope/gwcs/issues/152]