solving with normalized influence coefficients (viscoelastic)

In 10.1016/j.triboint.2017.01.032, the inner contact solve loop is
adimensionalized by the shear modulus (and assumes nu = 0.5). The information of
elastic properties is instead cast onto the rough surface (scaled by G).

To avoid messing with the user's definition of elastic properties in the model,
the influence coefficients of the solver's integral operator are modified with a
guard in the solve function, so their original value is restored when the solve
returns.