EnsightCombinedReader: Improve tetrahedron cells reading
The tetrahedron reading logic used a non-tetrahedron-specifc vtkUnstructredGrid::InsertNextCell
call which performed poorly due to a large amount of time spent managing a std::set
, and struggling with memory allocation previsions.
With this change we instead handle the cell ID uniqueness ourselves and use the tetrahedron-specific API, resulting in a 2~3x speedup on a large dataset.
The logic is also refactored a bit for readability