Break into multiple files, begin refactoring
Refactor image types around property dicts; add tests
Improve support of spatial dimensions

This switches storageorder -> spatialorder, which cleans up a lot of
things. This also greatly expands the set of supported operations,
including permutedims().
Make reading by ImageMagick more flexible

Still need to figure out how to extract the colormap, but everything else seems to be in place.
Document the new framework (partial)
Add a few new core functions
Beginning of reworked imwrite support