rs.utils.compute_structurefactor_multiplicity doesn't account for centering

[kmdalton]

Right now epsilon factors do not account for space group centering. We should change rs.utils.compute_structurefactor_multiplicity to account for the space group centering operations. This is easy enough to implement, but right now our test data for epsilon factors are from sgtbx. It is easily verified that these don't take centering into account.

>>> df = pd.read_csv("tests/data/sgtbx/sgtbx.csv.bz2")
>>> df.groupby('xhm').min()['epsilon'].max()
    1

I propose we modify tests/data/gen_sgtbx_reference_data.sh to use both gemmi and sgtbx for epsilons. When we test against sgtbx, we just have to remember to divide by len(spacegroup.operations().cen_ops) and/or epsilons.min().