I initially changed it to allow faster (flat) tracebacks *during*
front evolution, but it turned out not to be necessary. Putting back
to minimize the changes of this PR.
Installed a few hooks to allow more flexibility in subclasses, refactored
traceback() to allow obtaining tracebacks fast during front evolution, and
moved some initializations to reset().
Full list:
* integer division in calculating indices
* traceback_offsets use -2 for uninitialized, -1 for seed point. In that
we we can distinguish between these two cases in traceback()
* Clearing of traceback_offset, cumulative_costs, pushing start positions
into the heap, are now all done in reset()
* Reset is called every time that find_cost is called.
* Add hook _goal_reached()
* Add hook _examine_neighbor()
* Add hook _update_node()
* Replace while loop with a for-loop to prevent infinite looping in case of a bug.
* Split traceback() in two functions to allow getting a flat traceback fast.
traceback() calles these two private functions and behaves as just before.
Almar Klein