Elastic membrane

This PR adds the first solid mechanics in GRINS. This is for elastic membranes including large deformation (geometrically nonlinear). These are 2D manifolds. There is also ElasticMembraneConstantPressure that adds a constant pressure loading to the membrane (another geometric nonlinearity). There are two support examples - elastic sheet (also a regression test) and the inflating sheet. The ElasticMembrane Physics is templated around the constitutive equation. I went the CRTP route for designing the constitutive interface. Route now there's Hooke's law (because that's what I need) and incompressible Mooney-Rivlin (because there are quite a few benchmarks that use this).

There are no analytic Jacobians yet (they're started for the stresses at least) and also need to add post processing (stress, strain, etc), but I wanted to leverage #167 so I'm going ahead and making a PR for this and will add the other capabilities later.

Note the SolidMechanics variables has the extra stuff because I want to be able to reuse it for other classes (e.g. the cable Physics @cahaynes is working on) that may be 1D or 2D manifolds in 1D, 2D, or 3D space.