ENH: support mkdir as dictionary keyword

- this is a convenient means of creating an output directory from within
  the function object without corresponding python or shell script.

  Some pipelines (image generation) balk if the output directory does
  not exist. Others (vtm writer etc) will create their own.

STYLE: use OpenFOAM caseDicts config files, newer string expansion.

- Eg, "<system>/scripts" instead of "$FOAM_CASE/system/scripts"

- OpenFOAM CleanFunctions now include removal of the 'insitu' directory