Shut up some warnings
Wrong formula + minor flop optimization
Helper variables for readability
Reworked API for compute_stress
We need to plane stress incompressible hyperelasiticity specially

Incompressibility + plane stress implies a particular form of the
stress-strain law that doesn't come out naturally of the general
form. In particular, there's no dW/dI3 term (because it's zero),
but there's a contribution that need to be included because of the
incompressibility + plane stress conditions.
Build/use new plane stress/incompressible stress strain law

For ElasticMembrane at the moment.