egammaLayerRecalibTool: es2017 code from 21.2.

Port code from 21.2 to handle es2017_R21_v0 model.

Needed to fix EgammaAnalysisAlgorithms test failure in dbg build.

commit 2b13a41eaf5a9a1f514cc5ead11a3125547189e6
Author: Guillaume Unal <Guillaume.Unal@cern.ch>
Date:   Fri Jan 12 14:47:52 2018 +0100

   Implement "es2017_R21_v0" model
     - v7 MVA configuration as in previous R21 model
     - same systematic uncertainties as in latest 20.7 model (checked)
     - layer calibration from run 2 but modified to be applied on top of release 21 reconstruction (uses v5 version of egammaLayerRecalibTunes.root)
       Only applies E0 and E1/E2 correction (no need to 20.7 pileup correction and EMEC HV PS correction since this is already improved in the reconstruction)
     - no overall scale correction (but uses by default same smearing as previous R21 model)

commit 6cfea36ff5b35630caf6bfa13802dfc03c1747a5
Author: Ruggero Turra <ruggero.turra@cern.ch>
Date:   Wed Jan 10 17:35:01 2018 +0100

    align egammaLayerRecalibTool from svn