Change in ghost buffer handling in ghost cell generator

The way ghosts were exchanged was a bit too rigid. It was relying on a ghost buffer structure inside BlockT and using polymorphism, assuming that we could stack geometry complexity (from implicit geometry to explicit points to explicit cells). vtkUnstructuredGrid inner vtkCellArray structure is too messy to do it by hand, and should be done using a reader / writer, hence the impossibility to use polymorphism and add layers of complexity in the exchanged data.

The ghost exchange is now done "a la carte" for each input type, calling appropriate subroutines.

• Renaming ForGrids to ForStructuredData as it is more correct