Resolve Pyomo deprecation warnings

[lbianchi-lbl]

Resolves: #10

[lbianchi-lbl]

@bknueven when you have time, could you take a look at the test failures? e.g.

I suspect it might be due to me doing the naive switch between pyomo.core.base._VarData (now deprecated) to pyomo.core.base.VarData (and the same for ExpressionData and ParamData). The error is that component is None in that scope, but I'm not sure what could be causing this. I tried poking around with pytest -k test_parameter_sweep --pdb but without much success:

(the code is slightly different in that I extracted self._get_object() to its own line)

[lbianchi-lbl]

Replaced by #12.