Graph statistics

Improve current graph statistics builder:
- rename GeometryStatisticsGraphBuilderImpl to StatisticsGraphBuilderImpl
- add a generic frequency/histogram helper class
- add edge/vertex name/label statistics

I did not add the builder in the command-line parameters (well to be precise I added then removed it), as statistics are not very helpful for a non-developer, and adding the builder manually is straightforward. But I can re-add a command-line option if needed.