[spec] Required data for stops

[pietercolpaert]

We need to spec what a client can count of regarding stops.

Minimum:

• only an identifier

Next-level:

• also a WKT string
• also an rdfs:label

Optional:

• type of station
• ...

[julianrojas87]

Stops data already takes into account the semantics for a Stop, Station, BoardingArea and a GenericNode, as defined in the spec.

The relationships between stops an their parent station is also defined. See for example stop http://irail.be/stations/NMBS/008811007_14 at https://graph.irail.be/sncb/stops.