Some ancillary things to keep in mind:
- Reuse per-task information (which includes Evaluators now) (issue564)
- Can we enable reuse of heuristics across different searches? See issue121.

I moved some notes from issue559 about related issues here.

Once issue559 is merged, we should remove the traces of the old task transformation code. In particular this includes:
* Heursitic::convert_ancestor_state
* The transformation in Heuristic::set_preferred
* The check that the given task is a transformation of the root task in
  LandmarkHeuristic::initialize

Possibly this requires other changes such as
* making PerStateInformation no longer dispatch on the task
* transforming states in the virtual methods of ModifyCostsEvaluator
* updating the place the tasks are wrapped into the special axioms tasks (tasks::get_default_value_axioms_task_if_needed)