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W' expenditure and reconstitution calculator

Calculates the W' expenditure and reconstition during a ride.
The implementation is sub-optimal and will need to be optimised
and probably re-written to improve performance before v3.1 is
released.

This is based upon research conducted at the University of Exeter
by Philip Skiba, Weerapong Chidnok, Anni Vanhatalo, and Andrew Jones
and is used with permission.

For more information on the research please see: ""Modeling the
Expenditure and Reconstitution of Work Capacity above Critical
Power." Med Sci Sports Exerc 2012;:1.
Missed off previous commit.
Add W' to All Plot

Will plot the W' balance as another data series.
Add Minimum W' Metric

It TREBLES the amount of time required to refresh the
metrics, so will need to be optmised before 3.1 is released.

But it should only need to run once.

I've also added a 'RideMetric::Low' type which we could
also apply to weight.
Disable Minimum W' Metric

.. its too slooooow.

.. lets fix the WPrime performance issue before
   introducing any metrics on the back of it.
W' calculate starting from 0s

Instead of starting at 20 minutes, which was a function of the
code for calculating decay, i.e. an implementation nit, not a
physiological constraint.