Feature/chem solver

Add a Backward eulerian time integration scheme for stiff systems. This permits to avoid QSSA and strange numerical behaviour. The algorithm is based on Gauss-Seidel iterations until precision goals are reached.

Cases 1 and 2 have been re-run with this new scheme. Case 2 expanded due to improved numeric stability with this scheme.

This is a small step for Man, a giant leap for ToyCTM.