Replace interp1d #2394

[jason-rpkt]

• Closes scipy.interpolate.interp1d is discouraged #2394
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• Tests added
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The existing tests pass. I have added 2 in test_iam.py under test_iam_interp.

[cwhanse]

Reason for importing CubicSpline instead of using make_interp_spline(..., k=3)? The latter would clean up the case processing, I think.