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been replaced by the CrackSurfaceDensity class.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!