Enhance TCX export

Provide more data to Garmin Training Center and
also makes the export compliant  with the latest
TCX schema. It displays correctly in GTC.

As an avid runner and biker I use GTC to combine
the data from both sports for additional analysis.

I'm sure others would benefit from the additional
data in the TCX file when importing to other tools.

The following derived/computed metrics have been
added to the export:

   1. Added MaximumSpeed
   2. AverageHeartRateBPM
   3. MaximumHeartRateBPM
   4. MaxBikeCadence
   5. AvgSpeed
   6. AvgWatts
   7. MaxWatts
   8. HeartRateBPM defaults to 1* (if not present or zero value)

*Garmin training center won't display the HeartRate
charts if the tag isn't part of every TrackPoint and
greater than zero.