Add possibility for partial ECAL digitization (towards working HGCAL)

[cmsbuild]

Add in options to turn off EB/EE/ES in ECAL digitizer, default all on.
Also, some modernization of code.

This change is technical in nature.
Automatically ported from CMSSW_7_6_X #11784 (original by @lgray).

[cmsbuild]

A new Pull Request was created by @cmsbuild for CMSSW_8_0_X.

Add possibility for partial ECAL digitization (towards working HGCAL)

It involves the following packages:

SimCalorimetry/EcalSimAlgos
SimCalorimetry/EcalSimProducers
SimGeneral/MixingModule

@cmsbuild, @civanch, @mdhildreth can you please review it and eventually sign? Thanks.
@argiro, @wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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