Add elevation processing for GPX files

[ray66]

This PR adds support for elevation data in GPX files. There is a slight drawback for tracks without any elevation data. In this case a diagram with a straight line at 'null' is drawn. It would be better to just display a text like "no elevation data" instead.

[martin-ueding]

Thank you for this contribution!

When there is no altitude data, the corresponding time series data frame should just not have the "altitude" column. So we should add some more logic that checks whether we really want to have this column. We could use Series.any to check whether there are any non-zero and non-nan (None) elements in there:

df = pd.DataFrame(points, columns=["time", "latitude", "longitude", "altitude"])
if not df["altitude"].any():
    del df["altitude"]
return df

Do you want to make this change or shall I do that? I'm happy to do it, but I don't want to take it away from you 😊

[ray66]

Thanks for your answer. I prefer that you make the change.

[martin-ueding]

I've merged your changes and put mine on top. It will be available in version 0.20.0.

[martin-ueding]

Version 0.20.0 is released you can update and try it out. If the issue persists, please reopen this ticket.