DataBlockFactory broken for single images

[phyy-nx]

Regression from 1049af2 and found by an xfel regression test.

This code snippet reproduces the error:

import libtbx.load_env
import os
from dxtbx.datablock import DataBlockFactory

dials_regression = libtbx.env.find_in_repositories(relative_path="dials_regression",
                      test=os.path.isdir)

path = os.path.join(dials_regression, 'image_examples', 'LCLS_cspad_nexus', 'idx-20130301060858401.cbf')
datablocks_cbf = DataBlockFactory.from_filenames([path])
datablock_cbf = datablocks_cbf[0]
imageset_cbf = datablock_cbf.extract_imagesets()[0]
assert imageset_cbf.get_detector(0) is not None

Note I'm about to check in a re-write of the xfel_regression test that found the issue to use ExperimentListFactory instead of DataBlockFactory. That will fix the current cctbx_project test failures.

Possible routes forward:

• Put above test into an xfailing test, fix the underlying problem, remove the xfailing flag
• Fully deprecate DataBlocks, obviating this issue.

[phyy-nx]

Re-write checked into xfel_regression to use ExperimentListFactory instead of DataBlockFactory (link)

[graeme-winter]

+1 for retire datablocks properly

but should probably add an xfailing tests and assess how hard to fix meantimes