Functionality for plotting 2D data

[wsp-mbuckley]

Describe Changes

• Added functions for creating subplots
• Added dataclasses for storing plot data
• Added functions for creating scatter, scatter density and hexbin plots
• Added matplotlib stylesheet for TfN (WIP!)

Task Checklist

• [ ] Have unittests been added (testing individual functions and their edge cases)
• [x] Have integration tests been added (if applicable, to test modules of functionality)
• [x] Updated RELEASE.md to reflect changes in PR
• [x] Have new dependencies been added?
  • [x] Have they been added to either requirements.txt or requirements_dev.txt.
  • [x] Have they been added to setup.cfg

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[wsp-mbuckley]

@BenTaylor-TfN this should be ready for merge, we don't have proper unit tests yet but I've got a couple of integration tests. I've removed the TODO comments and added issues to the repo. Once merged I'll setup the release on GitHub and hopefully PyPi should pick that up