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

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[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.

[jsiirola]

Overall in good shape. Some minor changes... @eslickj, let us know if you want to make them or if you want one of us to cover it.

[eslickj]

@jsiirola, thanks! I should be able to make the changes.

[blnicho]

@eslickj I found a few minor things that should be fixed

[codecov]

Codecov Report

Attention: Patch coverage is 93.77163% with 18 lines in your changes missing coverage. Please review.

Project coverage is 88.67%. Comparing base (2739945) to head (12b84f3).

Files with missing lines	Patch %	Lines
...omo/contrib/cspline_external/cspline_parameters.py	87.94%	17 Missing ⚠️
pyomo/contrib/cspline_external/build.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3216      +/-   ##
==========================================
+ Coverage   88.65%   88.67%   +0.01%     
==========================================
  Files         882      887       +5     
  Lines      101133   101422     +289     
==========================================
+ Hits        89662    89933     +271     
- Misses      11471    11489      +18     

Flag	Coverage Δ
builders	26.61% <21.45%> (-0.01%)	⬇️
default	84.76% <93.77%> (?)
expensive	34.04% <21.45%> (?)
linux	86.12% <93.77%> (-2.28%)	⬇️
linux_other	86.12% <93.77%> (+0.02%)	⬆️
osx	75.98% <51.21%> (-0.08%)	⬇️
win	84.60% <93.77%> (+0.02%)	⬆️
win_other	84.60% <93.77%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.