Interactive help on using the xMat

First defined in PhD thesis from Schranz (2018)
Other publications include...
Expects the following nine material parameters:
- 1: $\varphi_c$ (rad),
- 2: $K_r$ (kPa),
- 3: $\xi$ ,
- 4: $\kappa$ ,
- 5: $e_{c0}$ ,
- 6: $c_e$ ,
- 7: $c_5$ ,
- 8: $c_6$ and
- 9: $c_7$
Uses two state variables:
- 1: void ratio
- 2: replacement stiffness
Void ratio has to be initialized as a state variable.