Modify fetch_field to allow search around a center with a radius as well

[talister]

Hi @mkelley I have created a branch and pushed code to my fork to allow fetch_field() to search around a center with a radius as well.
I have some additional code that came out of the CSS calibration work to filter sources down to a specific radius (since the field was much larger than the 0.5 deg allowed by the PS1 search) but wasn't sure if you wanted in this branch, a different branch or not at all... I also wasn't sure of the natural home for it (maybe in caviacat.catalog.Catalog() ?) since it operated on Astropy Tables to use later on with xmatch() and cal_color(). Let me know what you think and I can code (or not) accordingly...

Cheers,
Tim

[mkelley]

Great, this looks good and is working for me. However, it would be best if the logic were in the base class, and that the derived classes just take a single coordinate. I can leave that change to you, or I can merge this and finish in a new PR.