Replace eigen-quadprog

The current QP solver seems to have some problems.

First of all, the unit tests sometimes fail in a seemingly random way, suggesting issues in memory management.

Second of all, the simulation sometimes freezes altogether while using the QP solver. This must be a bug inside the solver itself.

I think I will copy some code from eigen-quadprog. But we should make a separate repo with that code, some day soon.

Otherwise, any other suggestions? qpOASES, for instance?