The 'action' defines how the block volume is represented in
the compound block volume stack.
'Add' creates a block volume which is positively selected
'Subtract' creates a block volume which represents a hole or
negative space in the overall compound block volume.
The relativity enumeration determines whether the
BlockVolume specified is positioned relative to the parent
compound block volume origin, or in absolute world space.
This interface defines an entry into the CompoundBlockVolume which represents a volume of positive or negative space.