NXmx: read data_scale_factor

[phyy-nx]

New parameter as of nexusformat/definitions#1343. This is not full support of the parameter or of data_offset, but it's one of the use cases needed, namely reading a single gain value

[phyy-nx]

Requires dials/nxmx#40

[phyy-nx]

Now includes a test using data from @biochem-fan (thanks!), derived from CXI.DB #221. This is SACLA MPCCD data reformatted as NeXus NXmx, adding data_scale_factor as a parameter to account for a gain of 10.

@ndevenish this will need a new build of dials-data and of nxmx for these tests to pass. Thanks!

[ndevenish]

Okay, this seems to be picking up the new nxmx now, errors of form:

>       detector = dxtbx.nexus.get_dxtbx_detector(instrument.detectors[0], wavelength)
E       TypeError: get_dxtbx_detector() missing 1 required positional argument: 'nxdata'

tests/nexus/test_build_dxtbx_models.py:275: TypeError

[ndevenish]

Otherwise, I think this is ready-to-go?

[phyy-nx]

Great, thanks!