Speed increase in ComputeAlignment.py, without use of scaler.

In ComputeAlignmet.py:
- do a check if the pixel map is already clustered, if so use
existing clusters, otherwise cluster this pixel map. This increases
the time of clustering by a factor of 10.
- improved sanity check of nevents -> n_proc
- also added two extra debug plots from FilterHotPixel
Merge pull request #13 from sredford/master

Speed increase in ComputeAlignment.py, without use of scaler.