Hotfix 380

[rueberger]

Adds warning when raised when a label interpolator is used with an image with a suspiciously high cardinality.

A potential problem: runtime

Counting unique voxels takes about 10s on a large image (1000 x 500 x 250), due to using the 'quick' and dirty method of len(np.unique(moving)). The warning only needs a rough estimate of cardinality though. Runtime could be significantly improved using something like hyperloglog or calculating cardinality for a random sample of voxels.

[cookpa]

I don't think the warning should include NearestNeighbor interpolation, as that's perfectly sensible to use with grayscale images and won't tie up the system for a long period anyway.

[gdevenyi]

Multilabel should probably just be removed as its just an inferior version of GenericLabel anyways...