Fixes to cosmic ray tasks and closes #76, closes #57, and closes #17

This is a number of minor fixes to the cosmic ray task to address issues #76, #57, and #17. 

In `cosmicray_median`, the background value has been made to match the documentation so that if None is given, the std is calculated.   

A test was added for gbox.

cosmicray_clean now returns a copy of the passed `ccddata` object with the data and mask values updated accordingly.