FrontierModel should have scope of entire search tree

[robfitzgerald]

in #350, the API for FrontierModel was modified to remove the tree. it was reverted to only include the scope of the current and previous link. however, certain desirable frontier modeling features like maneuver-based filtering are not possible without the full tree history.

In order to support maneuver filtering and other frontier restrictions that require more access to route history, we need to reinstate the tree as part of the frontier model API, in particular, reintroducing tree and direction:

    fn valid_frontier(
        &self,
        edge: &Edge,
        state: &[StateVariable],
        tree: &HashMap<VertexId, SearchTreeBranch>,
        direction: &Direction,
        state_model: &StateModel,
    ) -> Result<bool, FrontierModelError>;