Relax version constraint on CGNS library from system
The requested CGNS version when using a CGNS library from the system (i.e. not built by VTK), can be relaxed to 3.3.1. The CGNS reader code does not use any API that is not in that version, and when it does, it checks for the version.
This allows users of VTK that bring their own CGNS library to use anything >= 3.3.1.