Add warm start

[WimVanRoy]

• Added the warm_start functionality to the wrapper
• Added a .gitignore file for the automatically generated files
• Added simple tests for the warm_start functionality

[lostella]

Looks good to me, thanks.

We could test that the warm starting actually takes place, by e.g. initializing the iterates from a previously computed solution and checking that 0 iterations take place.

But this already looks fine.

[WimVanRoy]

Ok, I improved the testing but it turns out that if you re-create the model and reuse x & y, you only need less iterations (f.ex. from 10 -> 5). This is probably due to the stop criteria that are updated each cycle.
Only when you can reuse the same model-data, the number of iterations are equal to 1.

If you are happy with the changes, you may merge this branch...

[lostella]

Just fix the random seed, then we can merge. Thank you!