Slicing a reflection table drops the identifier map

[phyy-nx]

Found while working on #1858, in cctbx.xfel.merge we have a reflection table slice operation, but it turns out that drops the experiment identifier map.

Simple reproducer:

print(list(refls.experiment_identifiers().keys()))
refls = refls[0:10]
print(list(refls.experiment_identifiers().keys()))

Output

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
[]

I think this means that the slice code in flex.reflection_table needs to include a pass through the experiment map. It would like like this in python:

subset = refls[start:stop] # drops identifier map
for expt_id in set(subset['id']):
  subset.experiment_identifiers()[expt_id] = refls.experiment_identifiers()[expt_id]

[phyy-nx]

Note, part of cctbx/cctbx_project#822 can be reverted if this is fixed.

[graeme-winter]

@jbeilstenedmands this just came up in conversation as a "would be useful to fix" - if you have a moment