The CellSetExplicit and CellSetSingleType classes have an ivar that
marks the number of points. There were several instances of code
creating cell sets without specifying the number of points. This can be
very bad if subsequent code needs that information.
RemoveUnusedPoints contains a couple of worklets and some helper methods
to compact the points in a CellSetExplicit. This is helpful when you
have an operation that creates new cells but might not use all the
original points of the inputs.
