Add reports from pytest-mpl, changed setup and tox

[ruaridhg]

Closes #97

• 2D and 3D pytest-mpl reports added for histogram and slice with html summary fig_comparison.html
• matplotlib added to setup.cfg testing
• cp and ls commands added to tox and mpl-result-path set in tox

[dstansby]

We don't want to commit an individual report from a test run to the repository - the report should be generated during testing (either locally or on GitHub actions), and then we want to upload that as an artefact at the end of the GitHub actions run.

So I think the TODO here is (let me know if this doesn't make sense or you think otherwise!):

• Undo adding the report directory, and put back report in gitignore
• Add a step at the end of the GH actions run that uploads the whole report directory using actions/upload-artifact

[ruaridhg]

We don't want to commit an individual report from a test run to the repository - the report should be generated during testing (either locally or on GitHub actions), and then we want to upload that as an artefact at the end of the GitHub actions run.

So I think the TODO here is (let me know if this doesn't make sense or you think otherwise!):

• Undo adding the report directory, and put back report in gitignore
• Add a step at the end of the GH actions run that uploads the whole report directory using actions/upload-artifact

I was actually just thinking the same thing so will add the upload-artifact step

[dstansby]

Noice, looking good!