Bug in the phase-field degradation function

[ritukeshbharali]

A small bug is observed in the implementation of the Phase-field Element w.r.t., the degradation function (factor). In the code, factor = 1.0-phase*phase+self.k. Lines L86, L149. However, for AT1/AT2 phase-field fracture models, the degradation function needs to be, factor = pow(1.0-phase,2)+self.k.