Fixing problems with 1) the KKT formulation, 2) discretizer

In this PR, I did two main task:

1. Fixing a problem in the formulation of the KKT problem.
2. Describing the Euler discretizer as simpletic one.

Additionally, I included few other unit-tests:

1. Checking the integration values for a linear system.
2. Checking the Vx against the KKT Lagrangian multipliers.

The unit-test (2) tackles fully the point 2 from #36 (closed).