Upload example NeXus-like test image data

[benjaminhwilliams]

It transpires that support for NeXus-like data in DXTBX is somewhat patchy (see cctbx/dxtbx#206). We need test image data sets to ensure the following:

• We continue to support legacy Dectris Eiger data (is this what we refer to as 'nearly NeXus'?)
• We support up-to-date MX-style Eiger data (simple diffractometer geometry with detector on the beam axis).
• We support more complex geometry, such as collected on small-molecule chemical crystallography beamlines with three- and four-circle diffractometers, like I15 & I19 at Diamond Light Source.

[phyy-nx]

See also cctbx/dxtbx#93