Added code to enable the new XYZMolReader and GaussianCubeReader

@mhanwell I made the two Chemical Readers recently contributed visible to ParaView. No issue with the GaussianCubeReader2. The XYZMolReader2 is uncovering an issue with the Molecule Representation in ParaView. Since the new version of XYZMolReader2 exposes multiple timesteps, ParaView should in principle be able to animate the molecular display. The first step loaded works well, but changing the timestep causes all atoms to collapse into the same point. I believe this is an issue with the representation, since the reader itself, in VTK proper, can animate timesteps just fine.