Initial GOVSS implementation

Includes LTP, RTP, IWF and GOVSS metrics with PMC integration
I've tested a couple of runs, a 10k race and a 5x800x400 VO2max session, and they are pretty close to the values calculated by http://www.topofusion.com/govss.php
Caveats:
1) GOVSS and PMC integration are the basic metrics, Intensity Weighting Factor (IWF) it is an interesting metric.
2) LTP/RTP are exposed as metrics to facilitate debugging, but they could be hidden to avoid metric space pollution. 
3) Athlete's height is estimated from weight using Stillman average formula, but it should be pulled from athlete's data
4) Kinetic Energy contribution is disabled, formula in reference spreadsheet doesn't seem to be right.
5) Aero contribution calculation is based on reference spreadsheet but it should be verified.
Ale.