Improve search-related notifications to path-dependent evaluators
Changes related to issues #724 (and #718) concerning the notification to
path-dependent evaluators of search events such as the start of the
search or a state transition.
* get_involved_heuristics has been renamed to
* Most importantly, only path-dependent evaluators now register themselves
through this function.
* Notification functions have been pushed up the inheritance ladder
from the class Heuristic to the class Evaluator.
* The `heuristics` attribute in search engines has been renamed to
`path_dependent_evaluators` for the sake of clarity.