Expose DofMap::tabulate_local_to_global_dofs in Python.
Change std::vector<std::size_t> typemapfrom intp to uintp array.
Merge branch 'jan/local2global-python' into jan/tensor-layout-python
Add Python typemaps for SparsityPattern.

SparsityPattern ctor and init arguments changed from pass by value
to reference.
Remove superfluous assertion in PETScMatrix::init.
Fix bug in TensorLayout ctor.
More careful input checking in SparsityPattern.