Harmonize parameter naming for selections/filters

[JGuetschow]

Is your feature request related to a problem? Please describe.

Parameters to limit functions to a selection of the dataset use different names. In the downscaling function we use sel and in the aggregation functions we use filter

Describe the solution you'd like

Parameters that fulfill the same purpose should have the same name so users don't have to remerber different names for different functions.

Additional context

We've not used the new aggregation functions a lot, so it should be acceptable work to change the parameter name for these functions and adapt the code that already uses the functions.

[mikapfl]

I agree, it would be great to have a self-consistent API. We could also support the old name filter for legacy compatibility, but document that it is better to use sel instead.