Checks soft incompressibility on a single element under a homogeneous
deformation, for every 3D element type and every soft incompressibility
method. Under a homogeneous deformation the pressure field is constant, so
the nodal mean stress must equal the analytic material pressure and the
nodal energy density must equal the analytic strain energy density,
regardless of how the pressure is projected onto the element. This catches
the wrong pressure weight normalization for elements with more than one
pressure value per element (QuadhexElement, QuadwedgeElement).
The stiffness matrix is also checked against a numeric Jacobian of the
nodal forces, since the pressure weight matrix enters the tangent (via
Rinv) as well as the stress.