Two changes in ClusterEvent:

- added a check before clustering on the total number of hit pixels.
This should avoid trying to cluster large events where no triggers
were recieved.

- added a second sigma in the call to GetEtaCorrectedQWeightedCentroid
in line with the parameters this function expects.
Improvements to ComputeChargeDistance.

Made many simplifications:
- graph and histogram showed same data, removed graph
- define cluster for clarity
- use max(cluster.tot) in Qrel calculation
- fill the histogram straight away and return

Also added the check that the cluster aspect ratio is not 1.