Namespace for routes specification

It is important to define a proper namespace for the different properties and types.

One possibility is to define our own namespace, something like: `http://arquisoft.github.io/viadeSpec/` where we could define the new properties defined for viadeSpec.

Another possibility could be to define properties according to other formats like GPX and to adopt a more general namespace for them.

One possibility, suggested by @timbl [here](https://github.com/Arquisoft/viadeSpec/issues/2#issuecomment-594695302) could be to ask the W3C to create something like: `https://www.w3.org/ns/pim/gpx#` and to populate that namespace with the gpx properties.

No implementation files, tests, or entry points are named. Read the linked discussion in issue #2 and compare the proposed project namespace with the GPX/W3C alternatives; the work is done only after the namespace approach is agreed and documented.

Written by the indexing model from the issue text.