Support sparse matrices

[PABannier]

In high dimensional statistics, we usually rely on CSC and CSR arrays when matrices are very sparse. It is particularly for speeding up the coordinate descent procedure.

I can implement that in the ElasticNet subpackage if you find it worth it. What do you think?

[YuhanLiin]

The sprs crate implements CSR matrices and is already used in linfa-kernel and linfa-processing. Integrating it into ElasticNet would be welcome.