Altered FindMatchedCluster to consider track-pixel distances instead of track-cluster distances.

This is important because for large clusters (possibly delta rays) and for angled tracks,
the track position could be far from the cluster centre, but should still be close to
one of the hit pixels in the cluster.

In Cluster.py:
- new function GetPixelResiduals to get the minimum track-pixel distance for a cluster

In EudetData.py:
- alteration of FindMatchedCluster to use this new function

As a result of this change, more clusters are matched to tracks:
146264 (pixel matching) vs 146064 (cluster matching)
and the distance histogram is sharper.
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.
Merge pull request #10 from sredford/master

Altered FindMatchedCluster to consider track-pixel distances instead of ...