Allow resample output_wcs to be a gwcs object

[stscijgbot-rstdms]

Issue RCAL-909 was created on JIRA by David Davis:

Currently the resample parameter output_wcs is required to be a file. Setting this as a wcs object in a script causes

detector CompoundModel
    icrs          None  and data shape (3564, 3564)
----------------------------------------------------------------------
ERROR RUNNING STEP 'MosaicPipeline':
    Can't handle '  From     Transform   -------- -------------
    detector CompoundModel     icrs          None' as a file for mode
    'r'

To avoid having to write an intermediate file with the desired wcs it would be useful if reample would accept a gwcs object.