MPR mode is available for unreconstructable display sets

[swederik]

Currently the MPR mode is always available. This doesn't make sense in a lot of cases.

• If we have images of different dimensions within a display set
• If we have images with different numbers of components (e.g. grayscale and RGB) within a display set
• If we have images which are not in parallel orientations
• If we only have a single image
• If the SOP Class is not an image (e.g. structured report)
• If the SOP Class does not make sense to reconstruct (e.g. microscopy)
• Spacing between slices is not uniform (within a specific tolerance)

I'm sure there are more reasons why some stacks cannot be easily reformatted, and so we should add this logic somewhere. Slicer and MITK must have this logic somewhere which we can take a look at.