[Branch]: differential_rotation

This is the feature branch that contains some diff rot code, I think it needs some love so I am opening this so people don't forget about it!!
@wafels rather than forcing the user to pass in a Quantity in arcsec we should allow any unit that can be converted to arcsec. We can verify that a Quantity has been passed in then do `a.to('arcsec')` the resulting error from this is relatively useful, in that it says something along the lines of "Quantity can not be converted to Requested unit arcsec" iirc.