Interface the new solver with the toolbox

[agroudiev]

This PR adds support for the newton-sos solver. It can be enabled by setting the solver argument to newton-rs in ksos.solve.

To test the new solver, update the conda environment or install newton-sos with pip, and use newton-rs where you would use newton.

Known limitations:

• The kernel SOS solver might crash if the kernel matrix is not PSD. I will likely catch these errors properly before this is merged. (fixed)
• Support for Gaussian and Laplacian kernels only. (Feel free to open an issue here to ask for support of other kernels. If necessary, I can also implement a way to pass a custom Python kernel function as argument of the problem.)
• The X matrix (also called M) is not computed
• Phi and B are always computed, which we could probably avoid

[duembgen]

LGTM!

I also prepared for release 0.0.3, which we can create as soon as this is merged.