Wirc, working

[robertdstein]

This PR fixes many minor bugs, ensuring the WIRC pipeline works to produce stacks. It fixes a bug in flats.py, by which masking pixels in the flat also masked pixels in the science image. It adds image subtraction to WIRC by default, using WFCAM references. It removes the old/deprecated WIRC imsub code.

A couple of other minor changes:

• Adds a candidate configuration to search for candidates in a wirc difference image
• Introduces JSONLoader/JSONExporter processors.
• Minor fixes for parquet writing/loading.
• Adds ImageUpdater processor to copy info from source table into fits headers
• Checks every WFCAM image to see if it exists locally before downloading.