vtkConduitSource: Add state/metadata/vtk_fields node

When defining fields in a Conduit tree, it is often useful to include metadata about the fields that can be interpreted by VTK, and utilized appropriately. To that end, we have added a new node to the Conduit tree that can be used to store metadata about the fields in the tree. This node is called state/metadata/vtk_fields.

Each child of the state/metadata/vtk_fields node has a name that corresponds to a field in the tree. The number of children of the state/metadata/vtk_fields node should not match the number of fields in the tree since not every field needs to have metadata.

The current metadata that can be stored for each field is:

1. attribute_type: It is a string and represents the attribute type of a field that, so that VTk can use an attribute array, such as GlobalIds. The values uses are the entries of vtkDataSetAttributes::AttributeTypes + Ghosts. The complete list of values is:
   • Scalars
   • Vectors
   • Normals
   • TCoords
   • Tensors
   • GlobalIds
   • PedigreeIds
   • EdgeFlag
   • Tangents
   • RationalWeights
   • HigherOrderDegrees
   • ProcessIds
   • Ghosts
2. values_to_replace and replacement_values: These two are vectors of values that be used to replace specific values in the field. This is useful when the field has some values that are not valid for VTK. For example, the vtkGhostType array uses specific values that other ghost cell generator algorithms may not use. In this case, the values_to_replace would be the values that need to be replaced, and the replacement_values would be the values that should replace them. The two vectors should have the same length.

Example:

conduit_cpp::Node mesh;
CreateUniformMesh(3, 3, 3, mesh);

std::vector<int> cellGhosts(8, 0);
cellGhosts[2] = 1;

conduit_cpp::Node resCellFields = mesh["fields/other_ghosts"];
resCellFields["association"] = "element";
resCellFields["topology"] = "mesh";
resCellFields["volume_dependent"] = "false";
resCellFields["values"] = cellGhosts;

std::vector<int> cellGhostValuesToReplace(1, 1);
std::vector<int> cellGhostReplacementValues(1, vtkDataSetAttributes::HIDDENCELL);

std::vector<int> cellGhostsMetaData(1, 1);
conduit_cpp::Node ghostMetaData = mesh["state/metadata/vtk_fields/other_ghosts"];
ghostMetaData["attribute_type"] = "Ghosts";
ghostMetaData["values_to_replace"] = cellGhostValuesToReplace;
ghostMetaData["replacement_values"] = cellGhostReplacementValues;

Lastly, the usage of ascent_ghosts has been deprecated and should be replaced with the new state/metadata/vtk_fields node, as shown above.

cc: @christos.tsolakis @charles.gueunet @berkgeveci @cory.quammen @acbauer.