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.

![resy_s3x2y2_qweighted](https://cloud.githubusercontent.com/assets/8256377/4137935/35f1fe98-338c-11e4-840f-0d902f51fb25.png)
![resy_s3x2y2_etacorrection](https://cloud.githubusercontent.com/assets/8256377/4137940/3c1ab47c-338c-11e4-870c-00418f79e94d.png)