Gstreamer 1.0: merge !25 into gstreamer_python_upgrade

Related to #56 (closed)

Note this change is built on !69 (merged). If you'd like to review this in isolation, check the last 4 commits.

This includes !25 (merged) and !16 (merged), (!16 (merged) I did in 2 separate merges as I thought they were different. They're both very small changes)

I've rerun the auto formatter since postcoh.c had some indentation issues after the initial merge. It may have grabbed fixes for previous merges too. I'll run each time from here on.

I've done 2 new runs: /fred/oz996/tdavies/spiir_project/sources/testing/gout/py3/MR70_tests