PFEGamma Rewrite for CMSSW 70X

This is the set of changes that implements the rewrite of PFEGammaAlgo that truly merges the first steps of electron and photon reconstruction in the GED.
Please keep in mind that this change does not *yet* affect PF, since pfElectron & pfPhoton algos are still the primary reconstructions for those objects.

It presently affects the physics performance its output electrons with |eta| > 2.0, since there is a problem with the pf-photon reconstruction that now affects electrons too. This will be fixed in a future pre-release as it requires retraining of an associated MVA and some tweaking of the algorithm.  This does not affect the present performance of GsfElectrons and pfElectrons, as it is a parallel algorithm.

Computing performance-wise, this algorithm has the same timing as (pfElectronAlgo+pfPhotonAlgo).
It has been checked in an electron gun sample with pileup and uses very little time.

This was developed in parallel to:
https://github.com/cms-sw/cmssw/pull/120#issuecomment-21180476
and should not interfere too much.
The only problem may be the complete rewrite of PFEGammaAlgo, since there are some changes in PR-120 that operate on the old code base.