Clones of some IVariableValue implementations decompose to the corresponding NumPy type.

IVariableValue has a clone() operation. However, something about the implementation for at least:

- IntegerValue
- RealValue
- StringValue

causes the clone implementation to instead return an instance of the corresponding NumPy type instead of the ACVI type. BooleanValue is not affected. It's not clear whether it would happen to array types but inspection suggests they would be fine.

Inspect the clone() implementations for IntegerValue, RealValue, and StringValue, comparing them with BooleanValue and the array types mentioned in the issue. Reproduce the behavior and add or update tests so these clones remain ACVI value instances rather than corresponding NumPy types.

Written by the indexing model from the issue text.