Allow python bindings to load .mlimage files with varying voxel types

[hmeine]

I have gotten a merge request a while ago that achieves this, but

• it was for the sync branch, so it does not apply anymore, and
• more importantly, it makes the whole MLImageFormatReader and the PageIdxTable generic over the voxel type as well.

Instead of creating an enum of readers, I would like to have a get_tile() wrapper that looks up the voxel type and calls the (already generic) get_tile() method with the right type argument.

For that, it makes sense to parse the "ML_IMAGE_DTYPE" tag in the file header on load, and store it inside an enum field.

Probably, the MLImageFormatReader should have a getter method for the voxel type (much like calculateOutputImageProperties in the ML).

[hmeine]

I think I can close this issue as completed, even though some refactoring could be nice, but the functionality is there (for a seemingly reasonable subset of voxel types at least).