Compare QP solvers

[rhaschke]

• OSQP
• qpOASES
• qpsolver

Reuse existing QP formulation, which generates QP problem as H, q, A, lb, ub to form new problems via slicing:

• equality constraints: A[lb == ub, :]

[felixkol]

qpsolver wraps osqp as well as qpOASES while providing a standard interface for equality- and inequality-constrains. A slight modification to the current HQPMatrix will enable us to use the task stack with multiple solvers to compare them without implementing a matrix description for every single one.