support for Blueprint implicit topology, unstructured grids with no cells

to support Blueprint implicit topologies, https://llnl-conduit.readthedocs.io/en/latest/blueprint_mesh.html#implicit-topology, we propose the use of vtkUnstructuredGrids with zero cells. This is typical of particles-only simulations. A Conduit node has the following entries:

topologies: mesh: type: "points" coordset: "coords"