Topic diff xs sigmae

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.

Maybe we need also ig1 and ig2, that are passed per reference.