Added a skipCalculation and isCalculated to the ShortestPathBasedCentrality class to allow users to respect encapsulation. The skipCalculation also allows to fill the centralities via the processShortstPaths method.

Changed the list of centralities to calculate in ParallelCentralityCalculation to protected, allowing users to extend the class and retain functionality.

Also made use of the skipCalculation method instead of the package visibility *hack* with doneCalculation.

fixed my own fix.
Merge pull request #2648 from gsummer/2.1-maint

Fixes to the graphalgo facilities for 2.1 maint