ENH: rewrite of foamToVTK to include parallel output (#926)

- Default format is now XML binary (base64) instead of legacy format.
  The old -xml option is redundant and ignored.
  The new -legacy option can be used to force legacy output instead.

- Polyhedral decomposition is now off by default (old -poly is ignored).
  The option -poly-decomp forces decomposition of polyhedrals into
  primitive shapes.

- reduced memory footprint by reading and converting fields
  successively.

- Creation of symlinks to processor files is no longer required or
  desired. The old -noLinks option is ignored.

- Ignore -useTimeName option. Always number according to timeIndex.