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.

TODOs for final implementation:
1. Check that goalcount heuristic benefits from the changes in this issue.
Converting GlobalStates to States made it slower (see issue554) and the change 
in issue348 should imply that the performance goes up again if we avoid copying 
the state due to a task transformation.

moved TODO about checking for the performance of goalcount from issue559 to the summary here.

More things to cover in this issue:
* convert_ancestor_state and get_ancestor_operator_id
* update the comment in task_proxy.h (marked with "TODO(issue1208)")
* possibly the cost transformation in search algorithms

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)