Minimum Reachable optimization

[Kalashnikovni]

Currently the minimum reachable operation composes many times different maps. By profiling some examples, this last operation proved to be one of the most costly in regards to execution time. A very brief sketch of the proposed new implementation to be testes is as follows:

1. Keep track of MRV (minimum reachable vertex) with a representative's map Rmap, and the distance to each of them with a distance map Dmap. Also, a successor map Smap will indicate the path used to reach each MRV.
2. Calculate a new Rmap with adjacent vertices using minAdj.
3. Between all the possible successors that lead to the MRV, choose the one with minimum distance to the MRV according to Dmap. The distance is necessary for the case in which the graph contains cycles.