Streamline histogram and stats generation

[gillins]

This PR does a few different things:

1. Returns the created dataset from doTiledShepherdSegmentation_finalize/doTiledShepherdSegmentation so it is all ready to use by the caller.
2. Writes the Histogram to the RAT by default (we have to calculate this anyway for the missing segments check)
3. Calculates overviews in AWS Batch

[neilflood]

I think tiling.py line 688 now needs to pass outDs.

[neilflood]

Good work with the tests @gillins

I can see that this is because the file is now left open, since we are now passing the outDs back. However, I can no longer remember why we felt this was necessary. What would happen if we just left it as it was, and you re-opened it to do the histogram and overviews?

We can see from the way it scrambled the tests script that we have made a fundamental change to the behaviour of the API, and I don't remember why.