Load NXdetector pixel_mask into DataArray.masks

[SimonHeybrock]

See https://manual.nexusformat.org/classes/base_classes/NXdetector.html#nxdetector-pixel-mask-field. We should turn this into masks of a scipp.DataArray (the the detector is loaded as a DataArray).

Scipp only really works with boolean masks. Therefore, we should inspect the bitmask, and split it into individual masks. Only bits that are actually in use should result in creation of a corresponding mask.

[jokasimr]

I've started looking at this and I have some questions:

1. Do you have an example of a Nexus file that has the pixel-mask field assigned? I looked at one of the test cases from the scippnexus documentation but did not find the field:

   from scippnexus import data
   filename = data.get_path('PG3_4844_event.nxs')
   import scippnexus as snx
   f = snx.File(filename)
   detector = f['entry/instrument/bank102']
   list(detector.keys())

   ['azimuthal_angle',
   'data_x_y',
   'distance',
   'event_id',
   'event_index',
   'event_time_offset',
   'event_time_zero',
   'local_name',
   'origin',
   'pixel_id',
   'polar_angle',
   'total_counts',
   'x_pixel_offset',
   'y_pixel_offset']

2. How do we determine which bits are in use? If it is set to 0 in the entire array?