Ancillary variable aware cube arithmetic

[bjlittle]

✨ Feature Request

When performing cube arithmetic with one or more cube operands that contain Ancillary Variables, the resultant cube no longer contains any reference to the Ancillary Variables i.e., they are explicitly cleared from the resultant cube.

Extend cube arithmetic such that it maintains the Ancillary Variable metadata.

Reference: #3483 and this comment.

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