Ability to filter for specific routes during `RouteLayer.__call__`

[jamescalam]

The idea would be that when calling the RouteLayer we could set a decision scope by filtering for a specific set of routes. So, for example we may create a route layer containing routes ["politics", "chitchat", "AI", "compsci"] and for some queries we may want to search the full decision space, whereas others we might want to just search ["AI", "compsci"].

The way of supporting this in current version is to have two separate route layers, but this is inefficient with embeddings — so filtering may help there.