Add support for Catalyst steering extractors

Introduce extractors for Catalyst steering

Catalyst 2.0's "steering" capabilities can send data back to the simulation in order to "steer" it. To use the steering, the simulation used to send an XML file specifying a vtkSteeringDataGenerator proxy and properties to be filled by Paraview Catalyst.

While this still works, we introduce Catalyst steering extractors, a new kind of extractor acting as "sinks" in the visualization pipeline. Calling catalyst_results from the simulation will trigger these sinks, and their input dataset will be serialized to Conduit's mesh blueprint and sent back to the simulation in the results Conduit node. This special kind of extractor needs to be created in the Catalyst Python pipeline script. They are not associated to a trigger and a writer like other extractors do.

Example Catalyst pipeline script:

# Data is provided by the simulation.
producer = TrivialProducer(registrationName="grid")

# Perform a reduction on the input data.
sliced = Slice(Input=producer, SliceType="Plane")

# Create a steering extractor from the slice.
steering = CreateExtractor('steering', sliced, registrationName='steered_slice')

This script is executed as usual when catalyst_execute is called, but the extraction happens only when the simulation calls catalyst_results. This feature can be used to perform a reduction on input data, and get the result in the simulation directly.