Fix SPA syntax warnings. Fixes #472.

This is the minimal change set to fix the SPA syntax warnings by using a config.

I had to add a `similarity` function to `SPA` because the config object is only known to the `SPA` object. Thus, we have now two similarity functions (the other one in `spa.utils`). Not sure if we want to keep it that way or unify them somehow.

Also, we might add some syntactic sugar for accessing the vocabulary. Right now one has to do something like `model.config[probe.target].vocab` which does not seem very straight forward. Before it was just `probe.target.vocab`.