Support proper transforms for combined diffusion MRIs

[ReeceStevens]

Some diffusion MRI series combine several scans into a single series, with no acquisition number to distinguish them. This causes issues for dicom-numpy's generated image transformation.

Because slices are sorted by slice position, a diffusion series can appear to have multiple slices at the same location in space. This means the median space between slices will be 0, causing the image transformation to have all zeros in the slice axis.

To resolve this, slices should first be attempted to be sorted by instance number. If that fails, we can fall back to slice position sorting.