Cleanup `medsmaker_mocks.py`

[sweverett]

Right now, medsmaker_mocks.py has both some old legacy code and shortcuts that we should remove, fix, and / or improve. A few things to start with:

• Remove the option to select sources here & in the runner script, as this should always be done later. Optional arg, functions, etc. should all go
• PiffExtender solves the problem of MEDS expecting a PSFEx object, but we should see if there is a more "official" way since we do this in DES anyway. Will require Erin
• We want to edit these lines so that weights, masks, etc. are grabbed from extensions of the science images rather than separate global mask & weight files (as they will no longer be global soon; see #10 et al.)
• We need to save & pass the segmentation maps correctly so that the MEDS files can ingest them, and then inform the per-stamp mask making

[mcclearyj]

Another enhancement: there are values in the medsmaker/superbit/astro_configs/piff.config file that could probably be left blank, such as the input image file names. Suggest this as an enhancement.