Interface for setting postprocessed values?

[pbauman]

Right now, the API for compute_postprocessed_quantity (that is overridden by each Physics) is supposed to set the value for the quantity, if it owns it. However, I just noticed that I goofed and was initializing the value in each Physics, but you can't do that because if you set the value for the quantity in Physics1, but then Physics2 is called later resets the value to the dummy initialized thing. I'll fix this in a PR, but I think instead of relying on the user to reset the libMesh::Real, maybe it would be better to pass an object (PostprocessingContext?) where we call a set_value method or some such.