Improves performance of demo_stokes-iterative by switching from QMR
to MINRES method. There is no need to use QMR for symmetric problem.
Timings for serial python demo:
  TFQMR  54 sec
  MINRES 38 sec
including mesh generation and assembly.
demo_stokes-iterative: Use TFQMR method in case that MINRES is not availabe.
demo_stokes-iterative: fix in doc
Fix bug in SNES interface.
Fix bug in SNES tests.
Some assembler initialisation fixes to support some SNES improvements.
Eliminate some vector copying in SNES interface.
Fix typo.
Merge branch 'blechta/speedup-stokes-iterative'
Merge branch 'garth/fix-snes-interface-bugs'