The motivation for this refactoring was quite simple: amongst
all ancient (but renovated) and new hypetree grid filters, which
were part of the recently merged "HyperTreeGrid Version 2" branch,
the HTG CellCenters filter stood out as the only one which was
not deriving from vtkHyperTreeGridAlgorithm, which had also been
reworked entirely for this new version of HTG support.

This was because vtkHyperTreeGridCellCenters was, initially,
deriving from the vtkCellCenters class in Filters/General.
As VTK does not support multiple inheritance, it was left that
way but that was not consistent with our overall approach of
making all HTG algorithms derive from a single base class, in
the double goal of facilitating both the writing of new HTG
filters and their maintenance in the long run.

The current commit thus provides a re-writing of the HTG cell
center creating algorithm as subclass of vtkHyperTreeGridAlgorithm
while preserving the outside appearance of a vtkCellCenter-derived
filter in its API, specifically by introducing a VertexCells
instance variable in order to mimic that which is provided
in vtkCellCenters.

Note that no new unit tests are provided as part of this commit,
for the 4 existing ones for the previous incarnation of the HTG
cell center filter are still passing with unchanged results (as
they should indeed).