Compute effectivity for the reduced basis method

$$\eta_N(\mu) = \dfrac{\Delta_N(\mu)}{\Vert u(\mu) - u_N(\mu)\Vert_X}$$

Effectivity on the output (non compliant) :

$$\eta_N^s(\mu) = \dfrac{\Delta_N^s(\mu)}{|s(\mu) - s_N(\mu)|}$$