added Distributions as possible input in PostProcessor class
added Distributions as possible input in PostProcessor class

r29917

joshua-cogliati-inl/raven,joshua-cogliati-inl/raven,idaholab/raven,joshua-cogliati-inl/raven,joshua-cogliati-inl/raven,joshua-cogliati-inl/raven,idaholab/raven,joshua-cogliati-inl/raven,joshua-cogliati-inl/raven,idaholab/raven,idaholab/raven,idaholab/raven,idaholab/raven,idaholab/raven