Fits image processor

[jeffjennings]

• Adds a new script for IO modules, and adds the ProcessFitsImage class to it. This class has fucntionality to intake a .fits image, load and return it as a numpy array, and read the image extent and beam to return those.

• Adds a new test script with a test of the routines in ProcessFitsImage

I think this is the best approach to adding these functionalities into the codebase, as beam_utils isn't public, and anyway making it a dependency would inherit its casatools and similiar dependencies. I've very slightly modified the code for the functions from beam_utils. Most of this functionality is already in a tutorial, but the wrapping class in this PR is very convenient to load in a clean image for comparison. The class will be used in #211 to load a clean .fits image for comparison to cross-val MPoL results.

[jeffjennings]

Ok cool I see, thanks for explaining beam_utils - I didn't want to steal from it. And yeah, big reasons not to add those dependencies. Thanks for the review!