Interface to HiGHS as a QP solver

[cvanaret]

Add an interface to HiGHS as a QP solver for positive semi-definite Hessians. We'll then be able to run Uno's SQP method with convexification.

It was interfaced as LP solver in #72. For QPs, it's a bit trickier because HiGHS expects a compressed sparse Hessian in one of the following shapes:

• column-wise lower triangular;
• row-wise upper triangular.

At the moment, the CSC sparse format in Uno is only column-wise upper triangular (a la ASL). I need to write the transpose using @amontoison's function.