To keep conftest.py a more manageable size it would be good to split out the fixtures as suggested by Will in comment on #333

Not necessarily something to do here, but conftest.py is starting to get quite big with all the fixtures it contains. We could split out the fixtures into multiple files by getting pytest to discover them as plugins; causalprog does this for example, which lets us place fixtures inside tests/fixtures/**.py files to better split up the organization.

Could be particularly useful here so some of the more complex decorator-fixtures / data generation fixtures (in particular the so3 ones added here) can be moved into their own file, so they don't crowd out the more widely-used fixtures?

Originally posted by @willGraham01 in #333 (comment)