Refactor the various Level 2->Level3 funcs into a more centralized spot

[stscijgbot-rstdms]

Issue RCAL-835 was created on JIRA by Jonathan Eisenhamer:

In romancal.resample.resample, there are a number of places/funcs/methods that handle transferring L2 meta to the L3 output product. These are

• populate_mosaic_basic
• populate_mosaic_individual
• gwcs_into_l3
• l2_into_l3_meta

Nearly all of this work is independent of resample itself and are utilities/inherent component of the MosaicModel, much like the existing append_individual_image_meta.

Task would be to refactor all the various functions into a much smaller number of methods that are attached to MosaicModel. The code itself is removed from resample and resample just calls the MosaicModel api to accomplish the tasks. This refactor will also make the needed functionality more generally available without having to import resample to handle tasks that have nothing to do with resample