More flexible definition of units of measurement

[andrewlee94]

Derives from #650

So, to summarize the discussion:

1. To allow custom units in models, we would need to add convert statements to basically every term in every constraint in the core IDAES models. Whilst individually not hard, this is a large volume of work for which we currently do not have the resources.

2. Additionally, adding all these convert statements adds a very significant computational overhead in Pyomo at the moment. We have a propose solution to cut this down by adding unit_hints to named Expressions which we would trust for unit conversion and only check during consistency assertion, but this feature needs to be implemented in Pyomo, and then we would need to add these unit_hints in the IDAES code.

So, in short, we have a path forward, but we lack the resources to implement it right now, and it is probably not a short path.

If anybody thinks this is a higher priority, feel free to speak up, but we would need to find resources from somewhere.