Test incorporated and some documentation added.

Need to do a build-bot run with demo to test.

Ready for review

After running the basic ./bin/demo.sh, you can then run

python bin/check_astrometry.py output

to see the new output.

Comments on the PR. Many of them are quite minor nitpicks, but in general I think the code could use some tidying up both to better adhere to the coding standards and to make it easier to follow. It would be nice also to see a little more documentation – perhaps just a sentence or two in the README beyond describing what we're actually checking (although I realise that we don't do this for the other parts of the package), and some docstrings in the code describing what it's actually supposed to be doing.

That said, the basic logic seems fine.

Thanks John Swinbank for the helpful comments!

I've made the code better and better documented. I didn't incorporate everything, but I have your larger design thoughts in mind as I essentially revisit this code soon to generalize for validation tests.