Account for peridoic bcs in sparsity pattern

The demos for periodic and 'equality' bcs fail when Epetra is the default backend. With the present the design of these classes, it is not possible to make things work efficiently with the Epetra backend, and the present design is sub-optimal for PETSc. What is needed is that the boundary conditions be taken into account in the construction of the sparsity pattern. This will involve passing the boundary conditions as an optional bc argument to the assembler (and then to SparsityPatternBuilder).