Support for GPX

@timbl has suggested to include support for GPX as it is a popular format supported by lots of devices. In order to do that there are several possibilities to consider:

1. Create an RDF-based representation of the GPX format. I think the latest (1.1) specification is defined [here](https://www.topografix.com/GPX/1/1/gpx.xsd). One possibility which doesn't seem difficult is to represent that spec using ShEx.

1. Another possibility could be to translate a subset of the GPX format to Turtle. That's the approach followd by [this sed program](https://github.com/linkeddata/swap/blob/master/pim/gpx2n3.sed).

1. Finally, another possibility could be to include the raw GPX file as the value in a Turtle file that represents the routes and increasingly add more metadata to it. The solution could be to start with something like:
```turtle
:myRoute a viade:Route ;
viade:source [
viade:format viade:GPX ;
viade:value """
3.42016-06-17T23:41:03Z171
3.42016-06-17T23:41:13Z171
3.42016-06-17T23:41:18Z171
3.42016-06-17T23:41:23Z171
3.42016-06-17T23:41:26Z171
3.42016-06-17T23:41:29Z171
"""^^rdf:XMLLiteral
]
```

Start by reading the linked GPX 1.1 schema and the linked gpx2n3.sed example, then compare the three representation approaches described in the issue. The issue does not name repository files or tests; done requires choosing and implementing a defined GPX support approach with an agreed scope.

Written by the indexing model from the issue text.