Add bilinear basis function

[nataliakeles]

I have implemented a new basis function for the SyIdentPy library: the Bilinear Basis Function. This function is a subcase of NARMAX Polynomial Models.

For additional context, the Bilinear Basis Function is based on the method described in the following paper: Bilinear Basis Function Method - IJC 1989. This method is well-documented and has proven effective in various system identification tasks.

As the next step, I plan to develop comprehensive test functions to ensure the robustness and accuracy of the new implementation.

Please review the changes and provide any feedback.

[wilsonrljr]

Hey @nataliakeles,

Thank you so much for your awesome pull request! The Bilinear basis function you've implemented looks solid and is a great addition to the SysIdentPy. I really appreciate the effort you've put into this.

I've gone ahead and accepted the pull request since everything looks great on my end. The only thing left is to add some tests to ensure everything is covered. When you have the chance, it would be great if you could submit a follow-up pull request with the tests included.

Thanks again for your contribution. It's truly appreciated, and I'm excited to see more of your work in the future