Only load patch table when needed

[schlafly]

Currently the patch table is loaded whenever romancal is loaded, including for things like the ELP which do not use the patch table. The loading occurs whenever the module is imported.
https://github.com/spacetelescope/romancal/blob/main/romancal/patch_match/patch_match.py#L255

I believe the module is loaded whenever either pipeline is used, since the pipeline module pulls in the names of both steps.

romancal/romancal/pipeline/__init__.py

Lines 6 to 7 in b24429c

	from .exposure_pipeline import ExposurePipeline
	from .mosaic_pipeline import MosaicPipeline

We should instead only load the patch table when something actually uses it. I noticed this at the moment since I have a slow connection to /grp and couldn't figure out why it was taking a half hour to do something like strun roman_elp -h; unsetting PATCH_TABLE_PATH led this to instead only take a few seconds.