Simplify the calculation in mooneyrivlindensity.hh
Simplify the calculation in mooneyrivlindensity.hh: Do not calulate the eigenvalues anymore but replace it by a function of trace(F^TF). This reduces the assembly time a little bit.
Simplify the calculation in mooneyrivlindensity.hh: Do not calulate the eigenvalues anymore but replace it by a function of trace(F^TF). This reduces the assembly time a little bit.