Handle manifold cell connections for conversion

Reported in EP1653 by @Chiara

As @andy and @Mattijs noted and provided the basic "repair", the problem arises from how additional faces are added into the mesh for AMI. Neither VTK nor Ensight can assemble polyhedral where the faces are essentially duplicates of other ones.

The first course of action is filtering the mesh cells (for VTK and Ensight) to have unique connectivity. However, even still exhibits problems with volPointInterpolation.