HTG: Compute Visible Leaves Size filter
A new HTG utility filter for HTG has been added : `vtkHyperTreeGridVisibleLeavesVolume`.
This filter creates 2 new cell fields using implicit arrays under the hood:
- `ValidCell` has a (double) value of 1.0 for visible (non ghost, non masked) leaf cells, and 0.0 for the others.
- `CellSize`'s value corresponds to the volume of the cell in 3D HTGs, cell area in 2D or length in 1D.
`ValidCell` can act as a mask and allows, with `CellSize`, size computation aggregations.
Edited by Louis Gombert