[Development] Implement the possibility to specify an exponent for mesh stiffness based on element size

[sblauth]

We could implement an exponent which can be used to define the stiffness of the mesh (in the linear elasticity equations) based on the volume of the mesh. Currently, this is done by using

max_T vol(T) / vol(T_k)

for element k (the largest cells recieve stiffness of about 1)
We could add an exponent p so that we use

(max_T vol(T) / vol(T_k))^p

instead. The largest cells would still get a stiffness of 1, but the smallest would scale with (1/vol)^p, which may increase the mesh quality.