add a copy method to diffraction objects

[sbillinge]

Problem

That would be nice. Something like

def copy(self):
    return DiffractionObject(**self.everything)

where teat this as pseudocode....it is supposed to show intent, not how it would actually be coded.

usage would be that a test like this would pass:

copy_of_DO = DO.copy()
assert copy_of_DO == DO

Proposed solution