Tempus: Add StepperObserverComposite
Created by: ccober6
Add the capability to add a StepperObserverComposite, similar to those for IntegratorBasic and TimeStepControlStrategy.
@krcb has already has a prototype version with this patch against 6e8ea00769b3127490e00fb0f095401182b7d68b of EM-Plasma/Trilinos/develop