ParametricCoordinates::Vertices returns dax::exec::CellField.

Previously this method returned a dax::Tuple<dax::Vector3,NUM_VERTICES>,
which was harder to manage because you had to query the cell traits to
create one. Instead, just return a dax::exec::CellField class since that
is the type of thing it is made for.
Remove unnecessary argument of PointDataToCellData worklet operator
Remove unnecessary argument of CellGradient worklet operator
Minor comment fixes.
Merge branch 'exec-cell-cleanup'