ENH: WIP - Refactor the "WrapValue" methods in unit node

The name is changed to be clearer and more standard with regards to the
application's API.
Support for vtkVariant is added for a better interface with vtk.
The only exposed method now is GetDisplayTextFromValue(), the other methods
are protected as they are of little interest for the user.
All these methods are also virtual now to allow potential customization.

TO DO !
Make sure precision, min and max are included !

See Issue #1694