Feature suggestion: Include heart rate in statistics

[sgofferj]

GPX files can include the heart rate and the heart rate is a very good indicator for training efficiency. Based on age, weight and height, the heart rate gives a pretty direct indication of burned calories. It would be great to have the heart rate in the statistics including training zones (s. https://www.polar.com/blog/running-heart-rate-zones-basics/) and estimated burned calories.

Here's a GPX with heart rate data as an example (gpxtpx:hr tag): 2021-02-17 112405__20210217_1124.zip

Some formulas:

With VO2max:
Male: Calories/min = (-95.7735 + (0.271 * Age) + (0.394 * Weight) + (0.404 * VO2max) + (0.634 * Heart Rate))/4.184
Female: Calories/min = (-59.3954 + (0.274 * Age) + (0.103 * Weight) + (0.380 * VO2max) + (0.450 * Heart Rate)) / 4.184

Without VO2max:
Male: Calories/min = (-55.0969 + (0.6309 * Heart Rate) + (0.1988 * Weight) + (0.2017 * Age)) / 4.184
Female: Calories/min = (-20.4022 + (0.4472 * Heart Rate) - (0.1263 * Weight) + (0.074 * Age)) / 4.184

[sgofferj]

Related to #6

[theowoo]

Thanks for the app. A simple plotting of heart rate on the graph view would be much appreciated. Illustration: https://github.com/OpenTracksApp/OpenTracks/issues/1118#issuecomment-1031558060