Fixed thread-safety issues with ElectronLikelihood

[Dr15Jones]

Fixed const correctness problems with object directly and indirectly
held by ElectronLikelihood. Then enforced that only const functions
of the direct and indirect objects are called from const member functions
of ElectronLikelihood. This guarantees thread-safety.

[Dr15Jones]

As a bonus, I stopped the code from using large objects as arguments and return values. This should make it run faster.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_1_X.

Fixed thread-safety issues with ElectronLikelihood

It involves the following packages:

RecoEgamma/ElectronIdentification

@nclopezo, @cmsbuild, @anton-a, @thspeer, @slava77, @Degano can you please review it and eventually sign? Thanks.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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@nclopezo, @ktf you are the release manager for this.
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[cmsbuild]

+1
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-2814/457/summary.html

[slava77]

+1

for #2814 4bf88c3
jenkins OK

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_1_X IBs unless changes (tests are also fine). @nclopezo, @ktf can you please take care of it?