Descriptive names, inputs, outputs, and units for WCS transform objects

[stscijgbot-jp]

Issue JP-3607 was created on JIRA by Ned Molter:

The astropy GWCS module permits assignment of descriptive names of transforms, as well as descriptive labels for their inputs and outputs and default units.  These make it much easier to understand what a transform is doing, and which transform to use in what instance(s).  However, these are inconsistently used in assign_wcs, assign_mtwcs, and perhaps set_telescope_pointing.  For example, in assign_wcs.nircam.tsgrism(), the distortion model takes as inputs (v2, v3, wavelength, order) and as outputs (x, y, wavelength, order), but right now those are labeled ('x00', 'x10', 'x01', 'x11') and ('y0', 'y1', 'x0', 'x1') when calling transform.inputs()