Add label stitching functionality

[anwai98]

This PR adds a functionality to stitch labels to merge instance segmentations.

@constantinpape: Should we add tests for this?

[constantinpape]

@constantinpape: Should we add tests for this?

Yeah, it would be good to add a small test. You can do something quite simple: create a segmenation, e.g. via applying connected components to skimage.data.binary_blobs. Then add artificial tiling by adding id offsets to four tiles in the image. Then apply the stitching function and check that the result is the same as the initial segmentation.

[anwai98]

I added a simple test as discussed, and I think I can reproduce some issues I spotted (which is good to see already). I kept the test checks limited to avoid the obvious directions where the tests would fail (eg. matching the true number of instances with the merged instances)

[constantinpape]

The tests looks good. We can discuss the issues.

[constantinpape]

The tests fail because test data from EMBL owncloud currently is not available. The other tests pass now, so I will merge it.