Add multilinear regression

[insel-maz]

This PR introduces MathPHP\Statistics\Regression\Multilinear for performing multilinear regression. The change is intended to be non-breaking.

• The Regression constructor now additionally accepts a list of features (a list of x-vectors).
• MathPHP\LinearAlgebra\MatrixFactory::vandermonde() now utilizes the new MonomialExponentGenerator.

If you would like me to make any further changes, please let me know.

Thank you very much for your comprehensive library!

[coveralls]

coverage: 99.457% (-0.2%) from 99.66%
when pulling 1a4d424 on krumedia:multilinear-regression
into c064e80 on markrogoyski:master.

[markrogoyski]

Hi @insel-maz,

Thank you for your interest in MathPHP! And thank you for your pull request.

Please allow me some time to review the PR and understand the new functionality.

Thanks again,
Mark

[insel-maz]

Hi @markrogoyski,

I've just pushed a new commit to update some code to match the new shape of $points that I introduced earlier.
Do you think this new structure is suitable, or would you prefer a different approach?
Also, I'm wondering why the new unit tests aren't achieving higher code coverage. Any ideas?

Thank you

[markrogoyski]

Hi @insel-maz,

Apologies on the delay in reviewing this. It is on my to-do list. I will try to get back to you soon with feedback.

Thanks again for the contribution.
Mark