Fixes to the graphalgo facilities for 2.1 maint

The ParallelCentralityCalculation has (most likely accidentally) set the list of centralities to private instead of protected. This made it impossible to extend the class and override the calculate method, as the user could not gain access to the centralities.

Upon adding a centrality it also used the package visibility to set the doneCalculation flag in the ShortestPathBasedCentrality. While the user should not be able to mess around with that it also prohibits the user from recreating functionality like the ParallelCentralityCalculation via the processShortestPaths.
The proposed fixes to ShortestPathBasedCentrality aim to remedy this.