Add CF cf_role="timeseries_id" to station_id

[BGWKlein]

To improve CF compliance, the variable station_id should include the attribute:

cf_role = "timeseries_id"

according to the CF Metadata Conventions. This explicitly identifies the variable as the unique time series identifier.

Proposed new variable definition in HydPy:

    char station_id(stations=32, char_leng_name=12);
      :long_name = "station_id";
      :cf_role = "timeseries_id"

[tyralla]

I was not aware of that.

I guess we only add this information when writing NetCDF files. Or are there good reasons to query it when reading NetCDF files and raise a warning if it is missing?

[BGWKlein]

From my point of view, it is only relevant for writing. The only reason it would matter for reading is that HydPy searches for the variable defining cf_role if the variable station_id is missing in the input file(s). I wouldn't include this reading functionality to HydPy.