Better residue and error evaluation for convergence

Compute residue properly in R^3iCe

• need to be compute on both u and c

residu

• res_c=\frac{\partial c}{\partial t}-Wc+\lambda[...]-\frac{1}{\Gamma_{RX}}(c_0-c)
• res_u=loi\_de\_comportement
• lim=\Delta^i t \times max(x_{n+1},x_{n}) where i=2 because of BDF2 schem

error relative

• err=\frac{x_{n+1}-x_n}{max(x_{n+1},x_{n})} where n is the indice of the fix loop