Edited Cluster.py to catch the case where a cluster of size 4 is not shape 2x2.
Previously these clusters were missed, and a centroid position was not calculated,
so the centroid position would remain the initialised value.
Now the centroid position is calculated using the QWeighted method.
Merge pull request #2 from sredford/master

Edited Cluster.py to catch the case where a cluster of size 4 is not sha...