Heuristics now in flood_hillclimber.  They aren't used, but they are
set when instantiating the algorithm (and can be fed by the command
line).
removed pointer to current state.  The next set of states is now stored in frontier.  This should make it possible for future climbers to select multiple best nodes for diversity, or because we use multiple heuristics that disagree about what is best.
pushing the heuristics up a class -- they need to be called every time a state is generated or a node is opened in order to be correct.  By default, no heuristic will be supplied to random_flood, so it won't take much of a performance hit (an if-check), but things based on its flooding behavior will have appropriate heuristic calls without a shitload of code copying.
random_flood now calls reach_state where appropriate.  Landmark based heuristics can now be accurately computed on the frontier in flood_hillclimbing