Mooney-Rivlin membrane with plain stress assumption.
The membrane has a uniform initial thickness and obeys a hyperelastic material law with strain energy by Mooney-Rivlin: .
|dw_tl_membrane||(material_a1, material_a2, material_h0, virtual, state)|
fun is either weak_function or eval_function according to evaluation mode.
Evaluate stress or tangent stiffness of the Mooney-Rivlin membrane.
 Baoguo Wu, Xingwen Du and Huifeng Tan: A three-dimensional FE nonlinear analysis of membranes, Computers & Structures 59 (1996), no. 4, 601–605.