Cost Model Force Linear Cone

This creates a cost model which implements -> residue = A*f-ref. The idea is to use this residue and the activationmodelinequality to get Af<=0.

I have implemented a unittest for this cost:

• I have only implemented the Lx and Lu checking, since Lxx=rx*rx does not apply in this case.
• I have only implemented a simple checking of the derivatives, since activationmodelinequality is already being checked. So I don't check here which inequality is active.

This PR is related to #116 (closed).