Previously, DispatcherBase had an ivar to determine whether to use the
numInstances passed on the stack or to use a 3D range held in a
different ivar. This change allows either a 1D range or 3D range to be
passed on the stack, which I expect to be closer to how we we handle
this when 3D ranges are fully supported.

This also fixes a bug introduced with commit
fdac208a where the Use3DSchedule ivar
was not set correctly in UnitTestDispatcherBase.