Added EtaCorrection method for 3 hit clusters.

In Cluster.py:
- added eta correction method for L shaped 3 hit clusters
- this improves the 3 hit cluster resolution by about a factor 2 (see plots)

To do in the future:
- optimise the added fourth pixel tot (currently 1) to minimise the resolution.
May need to be a different value for different thicknesses. May need to be a
different value depending on tot of three other pixels.