[Feature Request] gemmification decorators

[kmdalton]

Decorators that scan the arguments of a function and automatically convert cell and spacegroup arguments to their proper gemmi types would be useful in writing standalone functions. We have 3 decorators

• a general purpose gemmify decorator. @gemmify scans for keyword arguments named sg, spacegroup , space_group, cell, unitcell, unit_cell, and converts them accordingly
• a spacegroupify decorator which has *args for each argument name to be converted

@spacegroupify("parent_sg", "child_sg")
def a_crazy_efx_function(parent_sg, child_sg, data):
    ....

• a cellify decoraor (in the same vein of spacegroupify)

[JBGreisman]

This decorator can then be added to:

• various functions in utils to avoid the need for argument checking
• DataSet.spacegroup setter method
• DataSet.cell setter method