The increment tolerance is used as an additional convergence
criterion.

Introduction of the new tolerance requires renaming of the
residuum tolerance.

The comment was blindly copied from previous implementation.

Use const references where necessary.

would otherwise prevent the use of dvalue for other models

set a fixed fracture normal vector or by default use
direction of 3rd principal stress vector as fracture normal

Drop unused commented variable.

based on Clausius-Clapeyron equation and corrected by Kelvin-Laplace
equation for curved menisci