NXmx: read data_scale_factor

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

newsfragments/XXX.feature

@@ -0,0 +1 @@
+Add support for reading the detector gain for nexus files

src/dxtbx/format/FormatNXmx.py

@@ -68,10 +68,11 @@ def _start(self):
  nxinstrument = nxentry.instruments[0]
  nxdetector = nxinstrument.detectors[0]
  nxbeam = nxinstrument.beams[0]
+ nxdata = nxmx_obj.entries[0].data[0]
  self._goniometer_model = dxtbx.nexus.get_dxtbx_goniometer(nxsample)
  self._beam_factory = dxtbx.nexus.CachedWavelengthBeamFactory(nxbeam)
  wavelength = self._beam_factory.make_beam(index=0).get_wavelength()
- self._detector_model = dxtbx.nexus.get_dxtbx_detector(nxdetector, wavelength)
+ self._detector_model = dxtbx.nexus.get_dxtbx_detector(nxdetector, wavelength, nxdata)
 
  # if the detector is between the sample and the source, and perpendicular
  # to the beam, then invert the distance vector, as this is probably wrong
@@ -86,7 +87,6 @@ def _start(self):
  if self._scan_model:
  self._num_images = len(self._scan_model)
  else:
- nxdata = nxmx_obj.entries[0].data[0]
  if nxdata.signal:
  data = nxdata[nxdata.signal]
  else:

src/dxtbx/nexus/__init__.py

@@ -245,6 +245,7 @@ def get_dxtbx_scan(
 def get_dxtbx_detector(
  nxdetector: nxmx.NXdetector,
  wavelength: float,
+ nxdata: nxmx.NXdata,
 ) -> dxtbx.model.Detector:
  """Generate a dxtbx detector model from an NXdetector and NXbeam.
 
@@ -461,6 +462,9 @@ def equipment_component_key(dependency):
  p.set_mu(mu)
  p.set_px_mm_strategy(px_mm)
 
+ if nxdata.data_scale_factor and not nxdata.data_scale_factor.shape:
+ p.set_gain(1/nxdata.data_scale_factor)
+
  return detector