writeTCX could deduct statistical values from the list of TrackPoints if they are not present in the TCXModel

[MrCsabaToth]

Values which could be calculated if they are not supplied:

• totalDistance (last reading in the last TrackPoint)
• maxSpeed (calculate across supplied TrackPoints)
• averageHeartRate (calculate across supplied TrackPoints, 0 values won't count)
• maximumHeartRate (calculate across supplied TrackPoints)
• averageCadence (calculate across supplied TrackPoints, 0 values won't count)
• calories (last reading in the last TrackPoint)

Potentially even totalTime, but we'd need to think about if that should include auto-paused periods or not. If not, then it could be just the last reading in the last TrackPoint.

[MrCsabaToth]

If we do these computations on-demand only, then all the APIs (models) will remain the same, just the library will be more intelligent. It will be able to even supplement few missing stats if the others present.