[vtkusers] vtkMolecule and vtkAtom

[Favre Jean]

Thanks David for the tip, and thanks to Marcus for pushing integration of this into the main code.

Things work really well in Python. I love the vtkSimpleBondPerceiver and vtkMoleculeMapper and the improved vtkPointGaussianMapper enables me to show the electrons through the molecular structure.

That Google Summer of Code project is a great contribution.

Jean/CSCS