Pgr_bdAstar accumulates heuristic cost in visited node cost

[AlexGacon]

Problem
A comparison of the pgr_bdastar algorithm with other astart implementation shows that the heuristic is not correctly used in pgr_bdastar: the current implementation for computing the cost of a visited node is to take the cost of the previous visited node and to add the cost of the edge between the two nodes. This is fine except for the fact that the cost of the previous node contains the heuristic cost, since it is provided by the queues.

The heuristic cost must only be used to decide which node to visit next and only for this: it must not be transmitted from node to node.

If you are ok with this analysis, I can work on a fix.

[AlexGacon]

Bug found when comparing the result of the A* algorithm with the bidirectional one:

In red the path with the classical A* algorithm, in purple the one provided by the bidirectional one. Both have been computed on a road network projected in Lambert 93; as a consequence a factor is applied to turn the distance provided the heuristic into a time.

The A* path is identical to the one found with the djikstra algorithm.