Generic DataProcessor to evaluate statistics

The idea is to generate e.g. 4 generic processors: MeanProcessor, StdProcessor, MinProcessor, MaxProcessor
that can act on all processors which have a "single" key, e.g. PedestrianIdKey or TimestepKey (Not TimestepPedestrianIdKey) and evaluate the statistics (min/max/mean/std) for over the key.

Further details

benefits: can be beneficial to avoid numerous mean / max processors as they already exist (MaxOverlapProcessor, MeanFlowProcessor,...) , useful for UQ since scalar qoi are easier to study, useful for repeated runs of simulation (simulationResult)