Add a function for cubic spline interpolation

[eslickj]

Summary/Motivation:

This adds an external function for cubic spline interpolation. I couldn't find this functionality in Pyomo, but I may have missed something. This is just a draft. I wanted to see if this is worth finishing. I can finish cleaning this up and adding tests or close it. I need functionality similar to this for pump and fan curves.

Changes proposed in this PR:

• Add an external function cubic spline interpolation function

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[mrmundt]

We discussed in the weekly dev call and expressed that this PR sounds interesting / useful. If you have time, @eslickj , please feel free to push forward with it.

[eslickj]

@mrmundt, Thanks! I think it's all there and functioning properly. I'll write tests as I have time.

[eslickj]

@mrmundt, I added tests.

[mrmundt]

@eslickj - The failing test is indicating that you have a missing __init__.py file. Please add that file in and ideally the tests should start passing!

[eslickj]

@mrmundt, I added the file.