Use CrackSurfaceDensity in BrittleFractureLocalOperator
The BrittleFractureLocalOperator class still used the old formula with 'psi_c', which lead to subtle bugs in various situations. In other parts of this module it has already been replaced by the CrackSurfaceDensity class. This commit completes the conversion of the module.
Warning: As some of the psi_c settings were computed incorrectly in the parameter files, this patch may slightly alter your problem specifications!