Rewrite of EtaCorrection method in Cluster.py.

In Cluster.py:
- use the possibility to call ErfInverse with a Qrel less than 0.5. The result will
be negative, which takes care of the direction of the shift.
- this results in much shorter code! And easier to follow I think.
- results in cluster centroid position are usually exactly the same. Rarely (13 times in
14k events) there is a difference in the ~15th decimal place of the computed centroid, due
to calling ErfInverse with a Qrel of eg 0.3 instead of 0.7. This makes no difference to
measured residuals.
- plots are the same as before
- time the code takes to run is the same as before

Also:
- removed duplicate code by calling existing functions
- added a check that the centroid of the cluster is always computed