Support structure factor cifs

[dennisbrookner]

At some point in the past I added support for mmcif files instead of pdbs, but I never added support for sfcif files instead of mtzs. This is trivial to do, via the rs.read_cif function.

Someday, (see: rs-station/reciprocalspaceship#296) I will just use the global rs.read function for this purpose. However, in the interim, I could potentially make some sort of little helper function:

def read_mtz_or_cif(file):
    try:
        return(rs.read_mtz(file))
    except RuntimeError:
        return(rs.read_cif(file))

The good news is that I don't think this is high priority, because it is trivial to convert an sfcif to an mtz. Among other options, you can open up an IPython shell, read in the sfcif, and write it back out as an mtz.