Fixed thread-safety problems with JetCorrectors

    Previously the JetCorrectors used the thread-unsafe class
    FactorizedJetCorrector to do the work. After refactoring the JetCorrectors
    now use the thread-safe class FactorizedJetCorrectorCalculator and its
    helper VariableValues class. The functionality is identical but the
    temporary variables now only exist on the stack.