Pass parameters directly to postprocessing

[emitra17]

Allow parameters of the PSet to be passed directly to a postprocessing script.
Current hack is to define a constant function in your model whose value is the parameter.

[wshlavacek]

Closing for now — the existing workaround (assign a global function the value of the parameter so it appears in the output file) seems to cover this. If there's a use case where that approach falls short, please reopen with details and we'll reconsider.