ENH: improved infrastructure for writing VTK content

Note: classes are prefixed with 'foamVtk' instead of 'vtk' to avoid potential
conflicts with VTK itself.

foamVtkCore
~~~~~~~~~~~
- General very low-level functionality.

foamVtkPTraits
~~~~~~~~~~~~~~
- Traits type of functionality for VTK

foamVtkOutputOptions
~~~~~~~~~~~~~~~~~~~~
- The various format output options as a class that can be passed to
  formatters etc.

foamVtkCells
~~~~~~~~~~~~
- Intended for unifying vtkTopo and PV-Reader code in the future.
- Handles polyhedron decompose internally etc

foamVtkOutput, foamVtkFormatter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Output helpers.
- Selector for individual formatters.
  Currently write all scalar data a 'float' (not 'double'). Can
  revisit this in the future.