Enable the gpx reader to use the declared namespace prefixes for recognized namespace...

Enable the gpx reader to use the declared namespace prefixes for recognized namespace URIs. (#990)

* use the declared ns prefixes in the gpx reader.

* remap other used uris, even though we have never

seen a different prefix used with them.

delete GARMIN_RTEPT_EXT, which was never used and appears to have
never existed.