ENH: code improvements for vtkCloud function object (issue #926)

- use parallel list writing, beginDataArray methods.

- use static_assert to restrict conversion of non-label integral types

- cache .vtp.series information by fileName instead of by cloud name.
  This issues if the output directory changes, and simplifies code.

ENH: emit TimeValue in files generated by vtkCloud

- additional information for passing to ParaView

ENH: vtkCloud output to postProcessing/ (issue #866)

- better alignment with other function objects, no collision with
  foamToVTK output.