Sliced ImageSets use too much memory

[phyy-nx]

At the moment, when creating an ImageSet that represents a slice of a multi-image datafile, the Reader object that is created to hold the arrays of dxtbx models is of size max(single_file_indices) instead of size len(single_file_indices). This leads to huge arrays with only one item for stills.

For example, if I have an hdf5 file with 10 images, and I an equivalent imageset for it, and I slice it like this:

subset = imageset[5:6]

Then single_file_indices will be the array [5], but the ImageSetData object will have a list of models with 6 items, the first 5 of which will be empty.