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