This patch introduces 3 new classes:

- vtkPeriodicTable: Convert between atomic number and symbol/name,
  lookup rgb values, lookup atomic radii, etc.
- vtkBODR: Container for arrays parsed from the Blue Obelisk Data
  Repository
- vtkBODRParser: Parser to read the XML files from the bodr.

It also contains bodr's elements.xml file and COPYING notice (MIT
license). These are installed to share/.

Change-Id: Ia7c928250e788695ad22a007dfdc9de4363c0474