Write into GPX files how the altitudes are calculated (EGM Correction and/or offset)

[Cat]

Currently there is no way to know which altitude has been written into a GPX files (Orthometric or Ellipsoidal).
It would be fine to add the information into the header comments.

[Cat]

Now we write the corrections applied to altitudes into the header comment section of GPX files.
For example, here below the header comment of a GPX file exported with EGM96 correction enabled and 10.5m of manual offset:

...
<!-- Track Statistics (based on Total Time | Time in Movement): -->
<!--  Distance = 135 m -->
<!--  Duration = 01:42 | 01:35 -->
<!--  Altitude Gap = 1 m -->
<!--  Max Speed = 6 km/h -->
<!--  Avg Speed = 4.8 | 5.1 km/h -->
<!--  Direction = SW -->
<!--  Activity = walking -->
<!--  Altitudes = Corrected using EGM96 grid (bilinear interpolation), -10.500m of manual offset -->
...