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.
Merge branch 'graphalgofixes' into 2.1-maint

small fixes to the ParallelCentralityCalculation and the ShortestPathBasedCentralities.