Add support for custom metadata to readpyaro

[dulte]

Readers from pyaro can add custom metadata, as the old pyaerocom reader could do. This is as of now not supported by the pyaro reader. The reader only saves a hardcoded set of expected data and metadata

Typical examples of such meta data is station_classification and area_classification, often used by EEA.

This is for now needed when reading the PMF data from Chen2022, which has the metadata station_type, which is similar to the EEA metadata.

It should be as easy as adding all "non standard" metadata entries as a key value pair to the ungriddeddata.metadata, as this is a dictionary

To start with, this metadata should be per station, and come from expanded versions of the Station class from pyaro

[dulte]

Dependent on

• [x] https://github.com/metno/pyaro/pull/25