Fixed thread-safety issues with ElectronLikelihood

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.
Merge pull request #2814 from Dr15Jones/makeElectronLikelihoodThreadSafe

RecoEgamma/ElectronIdentification -- Fixed thread-safety issues with ElectronLikelihood