Add a first working but untested version of support support for dP in the general dolfin Assembler
Only allow test and tral spaced with dofs only on vertices when assemble dP (point integrals)
Get assemble of dP (point integrals) over vertices working.
  -- vectors and matrices works in serial and parallel
  -- scalars need some more love. We need to figure out on vertex level who will
     have the responsiblility to tabulate it.
  -- Still no support for domains
Get subdomain assemble working for point integrals
Fix assemble of scalars in parallel
 -- only include contribution from vertex with the lowest rank from shared processes
Change interface to all help methods in FooAssembler to use
std::shared_ptr instead of plain pointers.
Fix init of mesh connectivity in SparsityPatternBuilder for point integrals
Tidy up assemble_vertices
Add py.test test for vertex assemble
Merge branch 'aslakbergersen/topic-change-unittest-to-pytest' into johanhake/add_vertex_assembler
Added test for error raising in system assembler for dP
Added demo which uses dP as source
Merge branch 'johanhake/add_vertex_assembler'
Move test to new position