Improve EinsteinHelfand class

[orionarcher]

This PR:

• Fixes a bug in the viscosity implementation.
• Adds a plotting function for the EinsteinHelfand class
• Allows automatic calculation of viscosities by specifying a fitting region
• Slightly improves documentation
• Adds data for a real system with known viscosity and tests against it (value is within 25%)

Fixes #53, #41 (partially), #28

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated? NA
• Issue raised/referenced?

[pep8speaks]

Hello @orionarcher! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2024-08-18 16:33:57 UTC

[hmacdope]

This has changed the expected value of the step traj test, if you can fix that up would be awesome.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 12 lines in your changes missing coverage. Please review.

Project coverage is 91.25%. Comparing base (f3dcaca) to head (c62630e).
Report is 10 commits behind head on main.

Additional details and impacted files