Add schema test for GPXData

It has some extensions to GPX that are readable by Strava.
Add byebug for debugging
Add GPX serialization for cadence

Uses Cluetrust GPX extension
Do not include element if not present
Add dist extension support

PWX may not always include it.
Add heart rate extension support
Add temperature extension support
Add power extension support
Generate time elements from time offsets

If the time element is not present in the PWX, then calc it and
add it to the parsed PWX document.