On-the-fly compression for dials.export of unmerged mmcif

[dagewa]

I guess if xia2 is going to write a compressed mmcif it would be make sense if DIALS writes the file as present, and xia2 does the compression?

Ideally, dials.export could compress on-the-fly depending on the file extension - e.g. if you pass mtz.hklout=foo.mmcif.gz
or mtz.hklout=foo.mmcif.bz2 then it would output a compressed file as appropriate.

Originally posted by @rjgildea in #1457 (comment)