Add a Green-Kubo viscosity implementation

[xhgchen]

The recommended approach for computing viscosity is the Green-Kubo method, making it quite desirable to have in this package. The plan is to provide a utility function to calculate shear viscosity from a NumPy array of the pressure tensor, as explained in Issue #9. Again, for GROMACS simulations, the EDRReader in MDAnalysis can obtain pressure tensor components, temperature, and volume directly from EDR files. For simulations from other MD packages, the user would input this information in the utility function.

Relevant best practices paper: https://livecomsjournal.org/index.php/livecoms/article/view/v1i1e6324

A more thorough write-up of this can be found on my personal website and blog at https://xhgchen.github.io/posts/gsoc-2023/