ViscocityHelfand Implementation in transport-analysis likely incorrect.

[hmacdope]

We have been going back through some results from a dependency of yours used to compute viscosities (transport-analysis) and are now pretty convinced the implementation of the Helfand method is likely wrong. We are working on a fix but unsure when it will be available. See here for meta-issue: MDAnalysis/transport-analysis#55

CC @orionarcher

[orionarcher]

Thanks for the ping @hmacdope. I'm the sole user of the viscosity implementation here AFAIK. I'll update the emmet codebase pending our fix / failure to fix the viscosity implementation.