Identify measurement stations globally by ID and operator

Measurement stations should be identifiable over harvesters to allow grouping of datasets created by the same station but provided by different entities. This is often the case e.g. for water measurement stations that are reported by a "Hochwassermeldedienst", and water quality data by another provider.

In the current implementation no harmonization of the operator is enforced, but this should be easy in a further step.

Only gwk_by is used as an exemplary data source atm.