Made apcor data optional for DownloadedFitsImages.  Trying to do
photometry calculations on them will just raise a ValueError.
Apcor files are now only downloaded for RealsWorkUnits.
Decreased max download threads to 3.
Added method to SourceReading for calculating the offset between 
their coordinate systems.
The image slice downloader now centers cutouts around the middle
observation of a source.
Reduced field of view to 250 by 250 pixels.
Merge branch 'master' into fov_centering
Merge branch 'optimize_downloads' into fov_centering

Conflicts:
	src/ossos-pipeline/ossos/gui/downloads.py
Merge pull request #85 from drusk/fov_centering

FOV centering