Improve Freud integration

[janbridley]

Description

This PR consists of a few QOL changes before the beta version is released to the group. It streamlines the export of boxes to Freud/HOOMD and adds conditional tests for interoperability with those packages.

I also properly doctested the RST source, using pytest-doctestplus instead of Sybil because I found it to be more straightforward. The doctest optionflags do not behave 1:1 identical to standard doctest, but the simplicity of this approach makes up for it in my estimation. I also improved on the test fixtures so the example_file.cif path is the same in all doctests.

Finally, I added sphinx-notfound-page and sphinx-copybutton, as it makes the docs a little easier to use.

• Add integration tests (Freud)
• Properly doctest QuickStart.rst
• Update CifFile.cell to return boxes in the [Lx, Ly, Lz, ...] format

Motivation and Context

Resolves #14

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality and should be merged into the breaking branch

Checklist:

• I am familiar with the Development Guidelines
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• I have updated the changelog and added my name to the credits.