Add specific docs for the GUI

[lukashergt]

This PR adds documentation for the use of the anesthetic GUI, and for how to use it when your data was not generated with PolyChord or MultiNest.

This came up in a discussion with @mj-will, who tried to apply the GUI to his samples, which are obviously not formatted in the same way as PolyChord or MultiNest.

To-Do:

• Add tests of the GUI with MultiNest data. Possibly worth waiting for add ultranest import #313 to be merged, which already modifies the various tests for different data inputs. --> already done in add ultranest import #313

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 anesthetic tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works
• I have appropriately incremented the semantic version number in both README.rst and anesthetic/_version.py

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a9e1ea5) 100.00% compared to head (6644f8f) 100.00%.

Additional details and impacted files

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Files Changed	Coverage Δ
anesthetic/_version.py	100.00% <100.00%> (ø)

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[lukashergt]

Check out docs here:
https://anesthetic.readthedocs.io/en/gui_docs/

[lukashergt]

@mj-will, is the blue box in these new docs helpful in getting the GUI to work manually?
Any suggestions on how to improve things?

[mj-will]

@lukashergt, I think the blue box looks good and the code works for me.

One thought, given how popular jupyter notebook/kernels are, you could add a note saying you'll need to use an interactive matplotlib backend (e.g. %matplotlib notebook) but that the user's mileage may vary (in my case it works but is pretty slow).