ENH: expose decomposePar -dry-run options -domains, -method

- can now drop older Test-decomposePar for exploration purposes
  and simply use -dry-run with the -domains and -method options.

- write VTK file instead of volScalarField in combination
  with -dry-run and -cellDist.

  Avoids adding any OpenFOAM fields and is usually faster to load.
  Also easier to rename than a volScalarField would be when exploring
  multiple decompositions.