Merge remove-center into Master.

Turns out getting rid of the `single_center` AstroDrizzle outputs was very simple. This just changes a few lines in `run_astrodrizzle.py` and gets rid of the center cfg files. Turns out I had already edited, in a previous branch, `test_imaging_pipeline.py` and `imaging_pipeline` so that the output dicts had only wide AstroDrizzle outputs in them. A bit of feature leakage between branches, sorry.