python support for labelMultiArray()

This PR adds wrappers for `labelMultiArray()` and `labelMultiArrayWithBackground()` for arrays of up to 5D in `vigranumpy/src/core/segmentation.cxx`.

I wasn't sure how to leverage vigranumpy's `multidef` function to wrap a function with support for several types *and* several dimensionalities; hence the explosion of `def`s.  If there's a smarter way to do this, let me know.