Finalize migration to Pyomo5 expression system

[whart222]

This ticket documents progress towards finalizing our migration to a new expression system.

[whart222]

@jsiirola @ghackebeil : I've added a branch for development to resolve this issue: new_expressions

I have also enabled PyPy test on this branch, whose failures should illustrate the changes that are needed.

[blnicho]

Pyomo5 expressions were merged in #272 so this can be closed