Migrate to poetry, fix readthedocs yaml, fix GitHub Actions and update README

[prakaa]

Changes:

1. Migrated to poetry for dependency management. setup.py has been removed, and support for Python 3.7 removed (unsupported now)
2. Add testing in workflow for pull requests
3. Read the Docs yaml has been fixed up - uses poetry
   • Untested because I don't have access to RtD management for nempy
4. GitHub actions implements matrix testing using poetry across all OSs. This appears to be working
   • Not implemented, but you can add an action to automatically publish to PyPI following a release. See NEMSEER example: https://github.com/UNSW-CEEM/NEMSEER/blob/137e37f449e1807c0ce20025390fb512f262f230/.github/workflows/cicd.yml#L129
5. Changes to README to condense information
   • Figure link is currently hard linked to transparent PNG which doesn't work well for dark themes. Best to replace with PNG with white background, and add relative link to joss-paper branch

Probably best to squash and merge if you accept this PR - lots of incremental commits

[mlee94]

Possible to remove pandas<2.0.0 in the poetry dependencies? New version of pandas introduces some good in-memory efficiencies. Hopefully doesn't break any tests or anything.

[prakaa]

Possible to remove pandas<2.0.0 in the poetry dependencies? New version of pandas introduces some good in-memory efficiencies. Hopefully doesn't break any tests or anything.

Yup good suggestion @mlee94. Have specified >= 2.0. Doesn't appear to break tests

[nick-gorman]

I've changed the plot on the readme to have a white background, but kept the hard coding for now.

[nick-gorman]

I've also added action based publishing to pypi