Apply changes from cms.Modifier immediately

As discussed in the Core software meeting, the cms.Modifier class now modifies the objects immediately if that cms.Modifier was passed to the constructor of the cms.Process instance.
Make error code to string map const

Made the std::map holding the translation from errors::ErrorCodes to std::string const by using the C++11 initializer list.
Added ModifierChain

Allow Modifiers to be grouped together in via a ModifierChain
Merge pull request #6295 from Dr15Jones/makeErrorCodesConst

Make error code to string map const
Merge pull request #6296 from Dr15Jones/haveModifierApplyImmediately

Have cms.Modifier apply immediately
Merge remote branch 'origin/CMSSW_7_3_X' into CMSSW_7_0_X