Make manoeuvre cost calculation happen during Dijkstra's algorithm runtime

[m-jeu]

When a user uses the application to search for efficient flights-plans, it computes all possible manoeuvres between orbits before actually searching using (the custom implementation of) Dijkstra's algorithm. Having orbits only compute their possible manoeuvres, with their costs when asked to by Dijkstra's algorithm has the potential to greatly improve memory usage by not having to compute manoeuvres for edges that don't need to be traversed anyways.

Make sure manoeuvres only calculate dV-cost once, and then return the saved number whenever asked.