Add gas station

[clange]

The basics can be found in

• schema.org
• linked geodata

We can introduce a new class, which is owl:equivalentClass to the above (and thus "inherits" all of the properties of the classes above).

The MobiVoc gas station and the existing MobiVoc charging point class should have a common superclass (e.g. "point of energy supply" – please make up a better name!).

The GoodRelations vocabulary may have further useful properties, e.g. payment-related ones. (There is no gas station in GoodRelations, but there are things that would be good superclasses of the MobiVoc gas station class.)