queries to timeseries are restrictable with "=" for all fields

e.g.

https://toar-data.fz-juelich.de/api/v2/timeseries/?var_id=19&sampling_frequency=Hourly

delivers all hourly time series of variable pm2p5 (AND).

Filtering should also work for lists, e. g.

https://toar-data.fz-juelich.de/api/v2/stationmeta/?var_id=19,7

delivers all time series of variable pm2p5 OR toluene.

This should also work in combination with #103 (closed) .