Model viewer -- Fix issue where expressions that evaluate to numpy scalars don't display right

[eslickj]

Fixes

If an expression value is a numpy scalar it did not display right in the model viewer.

Summary/Motivation:

After calculating expression values, they don't show up in the viewer is the values are numpy objects. This checks if the expression values are numpy scalars and if so converts them to int or float as appropriate. This checks the string class name, so I can avoid importing numpy.

Changes proposed in this PR:

• Fix model viewer display issue

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