dxtbx with Diamond nxs files: can access twice as many data images as present on disk

[graeme-winter]

Pointed out by @rjgildea

80 image grid scan as input:

dials.import xtal_3_2.nxs

dials.python -> 

from dxtbx.model import experiment_list
e = experiment_list.ExperimentList.from_file("imported.expt")
i = e[0].imageset
len(i) -> 80
data = i.reader().read(100)

does not fail - because this data includes a virtual data set at /entry/data/data which is pointing at internal references to /entry/data/data_000001 which is itself an external reference to another file which actually contains 80 images in this case. But the data are doubled as the VDS and external links are both counted.

In normal operation this is just funny - as in odd - because we always know the number of images, but it is still something of a sleeper which could catch us out later, in particular with more interesting uses of VDS.

Proposed fix: either read the VDS or reference the external data and don't do both