Add support for namespace

[NopMap]

I used the gpx plugin to create some GPX files with gpx 2.3.0. Unfortunately the tool EasyGPS rejected the files as invalid, as well as the Java GPX Library JPX.

The reason for this is the missing namespace, which seems to be a mandatory part of strict GPX 1.1 format:
<gpx [...] xmlns="http://www.topografix.com/GPX/1/1">

Please simply always add this attribute to the generated tag.

I noted there already is a PR #13 from 2021 adressing the same topic.