add CircleManifoldMesh (gdim 2, tdim 1)
add non-extruded circumference tests
add radial-extrusion-of-circle test
add poisson test
Merge pull request #331 from firedrakeproject/circle_surface_mesh

* circle_surface_mesh:
  add poisson test
  add radial-extrusion-of-circle test
  add non-extruded circumference tests
  add CircleManifoldMesh (gdim 2, tdim 1)