Support using Expressions in numpy matrix operations

[jsiirola]

Fixes # .

Summary/Motivation:

#2027 and #2070 added support for using variables and parameters in numpy matrix operations. Unfortunately, it failed to include Expression objects (which was recently pointed out by @eslickj). This PR resolves that oversight (including a simple test of the functionality).

Changes proposed in this PR:

• Support conversion of Expression objects into numpy.ndarray(dtype=object) objects

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