Optimise W' calculation

Instead of just reimplementing the spreadsheet we now
implement according to the original formulas as defined
in the W'bal paper.

This means there is no longer a 1200s limit to the decay
which was not part of the original paper.

We should be able to add some W'bal metrics now.
Add W'bal Metrics

Added;
- minimum W'bal == how deep did you go on the ride?
- max match == hardest sustained effort in KJ.

Fixes #760
Added W' Tau as a metric

.. so we can track and see if it correlates with anthing.
Added W' expenditure metric

.. energy spent above CP
Enable use of Minimum W' bal in data filter
merge upstream into master