Add side boundary conditions for the bottom layer only.
Extruded facet map computation occurs over a layered set.
Add offset to the extruded boundary node map.
Add strong side boundary conditions for extruded meshes.
Extend strong BCs to the LHS. Add new xfailing test for side bcs.
Eliminate some of the extruded mesh special casing.
Add exterior and interior _Facet object construction for extruded meshes.
Set up correct mesh size to have at least one interior dof and space to CG 2.
Add docstring for layers property.
Change test boundaries and update doc strings.
Change the strong bcs test case to use the initial expression.