Photon trees

This MR adds a new analysis for the photon CR, using trees. It is meant to be used with the statistical analysis.

It includes also "new" SF for photon ID efficiency in 2016. Only 2016 has been implemented.

With this MR, the tools to study the CR by itself are in place, but not yet the contribution to the signal region (computation of the weights) and the statistical part.

Namely, what is missing (and/or on which I'm currently working) are the following items:

• Computation of the weights for the number of vertices / pT reweighting. It's currently still done from the old InstrMetAnalysis class and the associated old scripts, this should be updated. The implementation of those weights in PhotonTrees is correct, but the methods are not elegant and should be updated.
• Everything to prepare the templates for the statistical analysis is not yet part of this MR, as it includes preliminary development and poorly-optimized code (to compute mean weights used for the transfer functions).
• Also note that launchAnalysis.sh is not yet meant to be run on trees (even if there are progresses). I should be used only with steps 1 and 2.