Convergence check

Finally, the $L_2$ norm of the conservative variables and their increments is calculated for the convergence check. Right now, convergence is reached if the norm of the change of the conservative variables does not exceed $10^{-8}$ of the norm of the variables themselves. Right now, the quotient of these norms is stored in file jobname.fcv (increment number, $\rho \epsilon _t$, $\rho v_1$, $\rho v_2$, $\rho v_3$, $p$ (incompressible) or $\rho$ (compressible), $\rho k$ and $\rho \omega$ (only for turbulent calculations) and the size of the fluid time increment, in that order).