Contact mechanics softening factor

[IvarStefansson]

In the contact mechanics equations, we currently compare tractions to c_num times (increments of) displacement jumps. C_num is by default computed as
c_num = shear modulus / (solid.residual_aperture() + solid.fracture_gap()) * SOFTENING FACTOR.

From a physical intuition, this resembles a Hooke's law, thus ensuring that the tractions and scaled displacements have similar magnitudes. However, the justification of the softening factor is truly heuristic ("making the fracture softer than the matrix " for the default value of 0.1). We should consider to either purge it or replace the shear modulus with the fracture's elastic modulus, i.e. the fracture stiffness parameter (which typically takes values roughly one order of magnitude smaller than the Lame parameters).

[IvarStefansson]

This is partly related to #957