Provide computational steering in ParaView/Live.

Allow vtkCPInputDataDescription grids to be marked as
steering parameters (i.e., they hold no visualization data
but are instead a table naming field-data arrays used to
steer the simulation).

When connecting live, ask the simulation to provide
steering parameters by default.
Do not make the user add them; the user may enable/disable
them by calling `coprocessor.EnableSteering(True/False)`
as desired.
When they exist,
create "pipeline producers" for them so they appear
in the pipeline browser.

This adds a new SI class for named properties used by
the steering parameters, which are properties added to
a self-generating source subclass.

See `Documentation/release/dev/` for
more information.