Interpolation unit Testing

[AndyAbok]

Thank you for contributing to FSharp.Stats. Please take the time to tell us a bit more about your PR.

This PR adds unit tests to the interpolation methods as mentioned in the Contribution guidelines on the coding conventions that unit testing is the biggest weakness this PR adds unit tests for the interpolation models.

Please list the changes introduced in this PR

• Add a couple of unit tests for some models.

Description

Adding unit tests for a couple of interpolation Methods in.

[Required] please make sure you checked that

• [x] The project builds without problems on your machine

[Optional]

• [x] Added unit tests regarding the added features

[bvenn]

Hi Andy, please add a little bit of context information to your PR. I've noted your new module wasn't included to the FSharp.Stats.Tests project, so I did it with minor further changes. Two of your tests are still failing and need some attention. You can run the tests using build.cmd RunTests