Refactor CellSetExplicit to remove NumIndices

VTK is in the process of refactoring vtkCellArray to move away from the existing design towards one that supports random access by introducing an offsets array:

// Current VTK:
Conn = { 3, x, x, x, 2, x, x, 4, x, x, x, x, 3, x, x, x }

// New VTK:
Offsets = { 0, 3, 5, 9, 12 }
Conn = { x, x, x, x, x, x, x, x, x, x, x, x }

This differs from the VTK-m CellSetExplicit storage, in that we store (numCells+1) offsets and do not have a NumIndices array.

We should change CellSetExplicit to match the new VTK layout to unify them and simplify zero copy. This will greatly reduce memory consumption and improve cache usage at the cost of an integer subtraction per cell access.

// Current VTK-m:
Offsets = { 0, 3, 5, 9 }
NumIndices = { 3, 2, 4, 3 }

// New VTK-m
Offsets = { 0, 3, 5, 9, 12 }

(Connectivity and type arrays do not change)