extensions with complex children are not parsed

[tstanis]

For example, consider these extensions:

<extensions>
  <gpxtpx:TrackPointExtension>
    <gpxtpx:atemp>29</gpxtpx:atemp>
    <gpxtpx:hr>89</gpxtpx:hr>
    <gpxtpx:cad>83</gpxtpx:cad>
  </gpxtpx:TrackPointExtension>
</extensions>

The problem is that GPXExtensionField assumes that all extensions are simple values, and doesn't parse the children:

https://github.com/tkrajina/gpxpy/blob/master/gpxpy/gpxfield.py#L254

I think the fix is to make the extensions into objects, but this breaks API compatibility as some users are expecting that extensions['myextension'] returns a string value, not an object. One way to deal with this would be to have extensions have a new field called subExtensions that contains the children.

What do you think?

[tkrajina]

I'll quote from README.md:

WARNING: The only part of the GPX standard which is not completely implemented are GPX extensions. The API for GPX extensions will change in future versions!!!

I was thinking on a customizable extensions helper which need to specify a serializer and underializer functions to user's own custom object. Then, those will be executed during parsing or .to_xml().

Everything not (de)serialized with this serialized will be in the existing extensions map. This will not break the existing versions.

Unfortunately, at the moment I don't have the time to implement that :( But, if somebody is willing to implement this, I'm willing to help/discuss/merge.