Add terms that appear as attributes

[labra]

It seems the initial conversion from XML ignored the terms that appeared as attributes like gpx:creator, gpx:lat, gpx:lon, etc.

The complete list is the following:

• gpx:version
• gpx:creator
• gpx:lat
• gpx:lon
• gpx:id
• gpx:domain
• gpx:href
• ' gpx:author`
• gpx:minlat
• gpx:minlon
• gpx:maxlat
• gpx:maxlon

Some of those attributes could be represented with properties from other namespaces. For example: gpx:lat and gpx:lon could be defined reusing the wgs_84 properties: wgs_84:lat and wgs_84:long instead, but in that case we should not use gpx:ele and use wgs_84:alt.

In my opinion we could just add those terms to the vocabulary and map between those terms and similar terms in other vocabularies later.