Max & Min altitude

[firefly-cpp]

Max and Min altitude were missing.

[vkurup]

Here's a question that I don't know the answer to... Is every TCX file guaranteed to have ns:AltitudeMeters? If not, or if the list can possibly be empty, then taking the max of an empty list will fail (and dividing by len(altitude) in altitude_avg will also fail).

Do you know what the spec says? It might be best to just assume that altitude_points can be empty, in which case we should check for that and return zero in these 2 methods and altitude_avg.

[firefly-cpp]

I have not seen a TCX file without altitude yet. In fact, I do not see any special problem here.
Nevertheless, I am going to add exceptions handling in the future. Moreover, there is a huge problem with heart rate values. Approximately 50% of athletes do not use heart rate monitor. We must add exception handling in that case.

[vkurup]

Here's an example: https://forums.garmin.com/showthread.php?220362-Export-to-TCX-Where-s-the-Elevation-Data&s=df7adbf1a3b64d568435d93c0967b1d0

My original files had altitude data, but when I exported them from that service, it had been stripped, so it would be nice not to break in those cases.

In any case, I agree with doing it as a separate PR. This one looks good. 👍

[firefly-cpp]

👍