Support filtering for entity in Dataset.pr.loc

[mikapfl]

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

To filter a dataset for the entity and other dimensions, you have to write:

ds[entity].pr.loc[{"other_dim": other_value}]

that's annoying because you have to treat entity different than other dimensions.

Describe the solution you'd like

We enhance the dataset loc accessor such that

ds.pr.loc[{"entity": entity, "other_dim": other_value}]

is valid.

Describe alternatives you've considered

Maybe we shouldn't do this because the return value of ds.pr.loc is not clearly defined anymore. If entity is in the filter, the return value is a DataArray, if entity is not in the filter, it is a Dataset. That's annoying because static type inference doesn't work anymore. So maybe we shouldn't overcomplicate things and live with the fact that filtering for entities is different from filtering for anything else.