Significant new vtkCellGrid functionality.

+ Add vtkCellGridCellCenters filter to compute cell centers as vertex cells.
+ Add vtkCellGridCellSource algorithm to create a single cell of the specified type.
+ Add an example application "CellGridSource" that demonstrates how cell-grid
  data is structured in memory and allows people to edit coefficients to explore
  function spaces.
+ Provide CPU interpolation that uses the same basis functions as GPU interpolation.
+ Add a legacy VTK reader+writer for vtkCellGrid so that ParaView client-server
  rendering works for small data (which must be transferred to the client).
+ Replace Intrepid basis functions with Intrepid2 basis functions.
+ Add a vtkCellGridToUnstructuredGrid filter; the output will always have linear
  geometry even if the source uses quadratic or higher order interpolation.
+ Add an HDIV attribute to the `dgHexahedra.dg` sample data.
+ Make the vtkPassSelectedArrays filter accept and process vtkCellGrid instances.
+ Add vtkCellGridTransform filter to apply a (linear) transform to a cell-grid's
  shape attribute.
+ Add vtkCellGridWarp filter to add an HGRAD cell-attribute to a cell-grid's
  shape attribute.
+ Add `vtkCellGridAlgorithm::GetInputCellAttributeToProcess()` to mirror
  `vtkAlgorithm::GetInputAbstractArrayToProcess()` (but for cell-grids); and
  `vtkCellGridAlgorithm::SetInputAttributeToProcess()` to mirror
  `vtkAlgorithm::SetInputArrayToProcess()`.
  Note that cell-attributes may currently only be referenced by name.
  Eventually, we should allow them to be referenced not by "type" (i.e.,
  field association) but by vtkCellAttribute::GetSpace().
+ Add attribute range information to cell-grid reader/writer.
+ When copying cell-grid instances, copy cached cell-grid range information in
  some cases.
+ Support range computation for HDIV and HCURL function spaces.
+ Add a vtkIOSSCellGridReader class that reads exodus files that end with a
  `.exdg` suffix as cell-grids instead of unstructured-grids.
  This also ports Spiros's performance changes to the new reader:
  changes from 9e3af159 through
  c0eca941 are included.
  Nodsets are not supported yet.
+ Add an option to the vtkCellGridReader/Writer classes to load/save
  JSON as binary message-pack data instead of plain-text.