Meta issue for using Evaluator instead of Heuristic in user code.

* Notify Evaluators about state transitions (issue724).

* Move heuristic progress statistics to Evaluator (issue750).

* Allow putting evaluators into documentation sections (issue749).

* Convert Heuristic options to Evaluator (issue766).

* Use Evaluator instead of Heuristic in option parser code (issue718).

* Rename "heuristic" to "evaluator" in variable and function names. Rename 
EvaluationResult::get_h_value to EvaluationResult::get_evaluator_value and do 
similar renamings for EvaluationContext (issue815).

* Let ConstEvaluator inherit from Evaluator (issue817).

* Clean up obsolete Heuristic forward definitions and heuristic.h includes
(issue819).

* Rename HeuristicCache to EvaluatorCache (issue821).

* Use --evaluator for evaluator predefinitions (issue822).

Once this is done, update issue207.

issue822 is now also resolved, which completes the work on this meta-issue.
Great work, everyone! :-)

Point 2. is issue821, which has been opened, resolved and merged.

All the subissues have been addressed. This does not mean that traces of the
Evaluator/Heuristic distinction are removed in the code internals, but that's
fine because that wasn't the goal. I had a look at which further changes we
might want to apply going forward:

1. Think about whether we want to keep the Heuristic class at all, and if yes,
for which purpose.

Conceptually, a "heuristic" is evaluator that is supposed to measure goal
distance, but the current distinction between when to derive from Heuristic and
when from Evaluator is mainly based on other considerations. We should think of
how to distribute the responsibility and whether we want a class named
"Heuristic" or whether it should have a name that reflects more closely what it
actually does. For example, I think it could be useful to have an abstract base
class for evaluators that are not composites of other evaluators.

This also affects the discussion of "Heuristic" in task_proxy.h

2. Rename the HeuristicCache to EvaluatorCache.

This is a simple thing.

I suggest we open new issues for these two things, deal with the second one
straight away, and deal with the first one later.

Add item "Clean up obsolete Heuristic forward definitions and heuristic.h includes." to summary.

Add item.

Add item.

Update summary.

Add item.

Update description of agenda item 6.

Move TODOs into summary field.

This issue intends to spread the use of Evaluator in search code and includes
following points.

1. Provide evaluator name for logging.

2. Add option to evaluators for enabling/disabling logging of better h-values.

3. Add option to evaluators for enabling/disabling counting of evaluations.

4. Notify Evaluators about state transitions (issue724).

5. Rename heuristic to evaluator in variable and function names.

With items 2. and 3., we intend to remove dynamic casts for checking if an
Evaluator is a Heuristic. Without the cast to Heuristic, Evaluator must provide
a name for logging (item 1.). Item 4. will finally replace all variables of type
 heuristic with variables of type Evaluator in the search code.