ElasticMembrane physics name
Changing solid mechanics variables interface slightly

Forgot to update this interface too
Accessors for solid mechanics variable names
Only need to look for FE_Family
Helpers in SolidMechanicsVariables to track if v and/or w are active

We need this because we can have domains that are lower dimensional
manifolds, but still have unknowns in the larger space. E.g. we can
a sheet in 2-D and 3-D even though it's a 2-D manifold. Similarly,
a 1-D bar can displace in 2 and 3 dimensions. But we can only determine
the manifold dimension, implicitly, at run time. Thus, we provide these
knobs to turn so that if the sheet is only in 2-D, we don't add extra
dofs for the z-displacement. This aspect not fully enabled yet in
ElasticMembrane, but this will provide the hooks for it.
Bug: grab w_var for the w_variable.
We don't want libmesh_not_implemented there.

Just don't do anything until we're ready.
First pass at SolidMechanicsBCHandling

Enough there to do displacement controlled problems. Will add load
controlled capabilities later.

Build SolidMechanicsBCHandling
Use SolidMechanicsBCHandling in ElasticMembrane
Add ElasticMembrane to PhysicsFactory