Make sure we use deviatoric strains when using compressible models

[finsberg]

For compressible models we need to make sure the strain energy function uses deviatoric strains (and not the total strain as in the current version). If we have

$$ \bar{F} = J^{-1/3} F $$

with $J = det(F)$, then

$$ \Psi(F) = \Psi_p(\bar{F}) + \Psi_c(J) $$

for compressible materials (here $\Psi_p$ is the material law and $\Psi_c$ is the compressibility penalty, while for incompressible materials we have

$$ \Psi(F) = \Psi_p(F) + p (J - 1) $$

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