Previously, the ArrayPortalCheck wrapper did not allow access to the
superclass' Get for 3D indices. This solves that problem and also fixes
it for Set (assuming there is ever an instance of that).
To get a portal to access ArrayHandle values in the control
environment, you now use the ReadPortal and WritePortal methods.
The portals returned are wrapped in an ArrayPortalToken object
so that the data between the portal and the ArrayHandle are
guaranteed to be consistent.
