Support providing a form to act as preconditioner

Often we know something about our problem that means we can construct a
preconditioning matrix that is different from the operator itself.
Support this in the variational formulation solve interface by allowing
the user to pass an optional Jp form used to assemble the
preconditioning operator.  If Jp is not provided, we default to using
the provided operator to construct the preconditioner as before.  Note
for consistency in naming, when building a LinearVariationalProblem we
use aP (rather than Jp).
Fix for issue #336 (incorrect ndofs for interior face loop)

Fix proposed by Lawrence: multiply number of dofs by two for interior facets.
Correctly attach nullspace to both mat and pmat
add CircleManifoldMesh (gdim 2, tdim 1)
add non-extruded circumference tests
add radial-extrusion-of-circle test
add poisson test
Add section in docs on providing pmat separately
Simplify pmat construction

Don't need a needs_pmat slot, just check if Jp is None.
Merge pull request #337 from firedrakeproject/fix_ndof_inparloop_over_interior_facets

* fix_ndof_inparloop_over_interior_facets:
  Fix for issue #336 (incorrect ndofs for interior face loop)
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)
Construct Mat Args using SparsityMaps where necessary

If the sparsity was built with restricted
domains (e.g. ON_INTERIOR_FACETS), we need to inform the Arg call when
build a Mat tensor argument, otherwise we get ALL (even though we're
actually not going to iterate over the whole map) passed in to the Arg
checker and we get complaints.  To fix this, build the appropriate Args
using SparsityMaps (as well as in the Mat construction and par_loop
iterate keyword).
Merge pull request #340 from firedrakeproject/moar_sparsitymap

* moar_sparsitymap:
  Construct Mat Args using SparsityMaps where necessary
Merge pull request #338 from firedrakeproject/separate_pc_form

* separate_pc_form:
  Simplify pmat construction
  Add section in docs on providing pmat separately
  Correctly attach nullspace to both mat and pmat
  Support providing a form to act as preconditioner
Fix up createSection calls in line with petsc4py changes

Number of fields is now determined directly from the length of the
numComp argument, rather than passed in separately.
Try old createSection calling convention first
Merge pull request #343 from firedrakeproject/petsc4py-compat

* petsc4py-compat:
  Try old createSection calling convention first
  Fix up createSection calls in line with petsc4py changes
specify gdim for extruded meshes
add cylinder Betti number and area tests
Fix LinearVariationalSolver docstring

solver_parameters, not parameters.