report timings: port and test

[oliver-sanders]

• Port onto latest pandas and matplotlib (bump pandas dep).
• Remove back-compat code.
• Add basic tests.

Report timings was removed from the cylc-flow Conda feedstock due to Python version incompatibility with the "pandas v1" dependency. It has essentially been "frozen out" by dependencies since then, this unlocks it.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

I'm happy with. And in the process I have a follow on which allows outputting to csv and json for about 10 lines of code.

Please don't forget the changelog entry