Make calcSampleDifference a protected member of extractorDiffXSec
Introduce (dummy) getReco function for DiffXSec just returning the
input value. Can be overloaded by child classes (virtual)
Add new class extractorDiffXSecPrediction derived from extractorDiffXSec
to be able to shift MC up and down according to Q^2 & M errors
Introduce new JES uncertainties in helper functions. Not used yet
Merge branch 'master' of github.com:simonspa/topmassextractor

Conflicts:
	utils/helpers.cc