ExternalGreyBoxModel: Implement hessian methods on base class

[michaelbynum]

Summary

In pynumero/interfaces/external_grey_box.py, ExternalGreyBoxModel works both with and without hessians. To use hessians, a derived class must implement a few methods as described in the doc string. However, there is not a default implementation of these in the base class with NotImplementedError. Then, the _ExternalGreyBoxAsNLP uses hasattr to see if hessians should be used (

pyomo/pyomo/contrib/pynumero/interfaces/pyomo_grey_box_nlp.py

Line 578 in e75a1be

self._has_hessian_support = True

). We should do this differently (e.g., try/except, has_hessian method, etc.)