Replaced u.rad**2 with u.steradian. Just a style thing.
Migrated back to feeding 'K' data into ppv_catalog.
Fixed insidious typo. Now pressure is pressure, not virial.
Added a couple of imports for interactive convenience.
All unknown distances are now "nan".
Added a print statement so I could see how many beams per pixel.
Added a couple of comment lines to help clarify some math.
If the downsample factor is 1, don't call resample
Defined perseus_demo.