Fix feather vs mosaic workflow issue in postprocessing

[astrojysun]

Added some workflow control code to ensure:
(1) the first round of feathering is only applied on individual parts (i.e., not full mosaics),
(2) the mosaicking step is not done on unfeathered data when feather_before_mosaic=True,
(3) when feather_before_mosaic=True, the output image has the right naming convention to be picked up by the following task in the pipeline (i.e., cleanup).

Also removed the redundant feather_after_mosaic keyword.

These edits shouldn't affect most existing use case where the feather_before_mosaic keyword is left as False by default. But they should allow end-to-end processing when it is set to True on purpose.