Partial correlation matrix implemented in signal model
Implemented split sigmE/E EB vs EE
for diff-xs: 5% uncertainty on EB, 10% on EE

Improved speed for diff-xs signal workspace creation:
* Removed systematics from nominal signal model ws.
* Added cacheing of gen-level bin.
* Avoid recomputation of sigmaE/E decorrelation.
Merge pull request #9 from musella/topic_diff_xs_sigmae

Tweak sigmaE/E variations