RZ mesh support

[JustinPrivitera]

Need to add RZ mesh support to Ascent. A user reported the following:

[Error] Ascent::execute 
file: …/conduit-0.9.2/src/libs/conduit/conduit_node.cpp
line: 14396
message: 
Cannot fetch non-existent child "x" from Node([0]/coordsets/coords/values)

The mesh is:

coordsets: 
  coords: 
    type: "explicit"
    values: 
      z: [-2.0, -1.0, 0.0, ..., 0.0, 1.0]
      r: [0.0, 0.0, 0.0, ..., 4.0, 4.0]
topologies: 
  mesh: 
    type: "structured"
    coordset: "coords"
    elements: 
      dims: 
        i: 3
        j: 4

There are conduit examples of RZ meshes so test data will be easy to provide.