Refactor waterdynamics to use UpdatingAtomGroup and add benchmark.

[hmacdope]

Is your feature request related to a problem?

The waterdynamics code currently makes a new AtomGroup for every frame, which is possibly inefficient and isn't in line with the newer analysis modules.

Describe the solution you'd like

We can refactor to use an UpdatingAtomgroup and streamline some of the loop structures to use array operations.

A benchmark would also be needed to assess any possible increases in performance.

[IAlibay]

I'm going to tag @alejob here. Given waterdynamics doesn't use the latest AnalysisBase formalism, maybe an overall target to update the whole thing might be a good idea?