Shashlik Fixes -> re-add ECAL driven seeding + split off mustache photons

[lgray]

This PR turns the ECAL driven seeding for Shashlik (and HGCAL) on again.
It was somehow turned off in a few previous patches.

The photons collection is now split between "photons" (old clusters reco) and "mustachePhotons" which uses the output of the mustache super clustering. Only the latter contains photons in the shashlik device.

This can be put in a patch release since it only edits .cc and python.

@nancymarinelli @dbenedet @matteosan1

[cmsbuild]

A new Pull Request was created by @lgray (Lindsey Gray) for CMSSW_6_2_X_SLHC.

Shashlik Fixes -> re-add ECAL driven seeding + split off mustache photons

It involves the following packages:

RecoEgamma/EgammaElectronAlgos
RecoEgamma/EgammaElectronProducers
SLHCUpgradeSimulations/Configuration

@civanch, @nclopezo, @mdhildreth, @cmsbuild, @StoyanStoynev, @slava77, @Degano can you please review it and eventually sign? Thanks.
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@fratnikov, @mark-grimes you are the release manager for this.
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[mark-grimes]

merge