Cannot "initialize" value in the postprocessing computation

See discussion in #175. TL;DR - was "initializing" the value in the postprocessing computation, but we can't do that because we might overwrite a value set by a previous Physics. #175 discusses redoing the API so folks don't make that goof in the future.

Merging to 0.5.0-release branch, but will manually merge to master immediately.