Add update_Q_A to MEX interface

[alexandre-faye]

Adds the update_Q_A method to the MATLAB MEX interface since I use it a lot.
This code works in my limited testing for my limited use-case.

[tttapa]

Thanks a lot for the contribution!
I'll have a detailed look and merge it next week.

[alexandre-faye]

I should precise something, if you still plan on doing the review: I implemented the MATLAB interface such that it takes whole matrices (either dense or sparse), whereas the C/C++ interface needs the upper diagonal coefficients of Q.

I feel like it fits the MATLAB style more, but it is sort of inconsistent with the other interfaces. For reference, the MATLAB OSQP interface requires the upper diagonal coeffs, but their update function is a bit different (in the lines of solver.update('Qx', Q_coeffs)).

In the end I also did this because it felt more fit to my use-case, but I'm open to doing it differently.