First draft of postProcessing script

First step along the lines of #154 (closed)

This introduces a new EasyjetPlus package to be used to host in particular the base classes Processor.h and IPostProcessTool.h.

Processor.h is used as main steering algorithm, accessing the input tree, creating the output tree, configuring the I/O for the branches, looping over the events and computing the new variables.

The computation of new variables is handled by dedicated tools based on the IPostProcessTool interface.

Input and output variables are carried over using maps with dedicated enums, defined in Utilities.h

Examples of specialisations for the post processing tools are provided with DummyPostProcessTool and SumOfWeightsTool. Analysis-specific versions can be hosted in the analysis packages, while tools of common usage, like for the sum of MC weights or cross-section can be kept in EasyjetPlus.

The example can be run at the moment with

athena EasyjetPlus/sowPostProcess.py - --inFile /atlas/strebler/HHbbtautau/ttHH_test/run/test.root

This can be considered as a starting point to develop a tool for the sum of weights and another one for the cross-sections for instance.

FYI @lderamo @mfujimot