Add W'Bal plot for swimming activities

Now that CP and D' parameters can be set for swimming activities, it would be nice to add a W'Bal plot, like the running and biking one.

This requires some thought, as lap swimming activities don't have 1 second samples (or close) like the run and bike activities have, instead they have one data point for each length.  The W'Bal plot would look nicer if it was sampled at 1 second interval.