Decomposed::DamagedElasticEnergyDensity class does too many
- It implements the St.Venant-Kirchhoff density in terms of the strain eigenvalues, split into tensile and compressive parts as described by Miehe, Welschinger, Hofacker.
- It degrades the tensile part with a hard-wired degradation function.
- It transforms the degraded density and its gradient and Hesse matrix from spectral to physical coordinates.
- It contains a part of the AT1 damage functional.
This merge request modularizes it.