Interactive help on using the xMat

Defined in publication from Kolymbas (2015)
Expects the following seven material parameters:
- 1: $\varphi_c$ (rad),
- 2: $c_2$ ,
- 3: $c_3$ ,
- 4: $c_4$ ,
- 5: $c_5$ ,
- 6: $e_{c0}$ and
- 7: $e_\mathrm{min}$
Uses two state variables:
- 1: void ratio
- 2: replacement stiffness
Void ratio has to be initialized as a state variable.