This object constructs a set of field tables that allow for multiple field variables to control the integration through a volme.
The integration through a volume typically only utilizes a single field variable (for instance, Density) to set up and control the values returned at the end of the integration. For things like isocontours, this is fine. However, more complicated schema are possible by using this object. For instance, density-weighted emission that produces colors based on the temperature of the fluid.