Construct k-local ProductDual instances from 1-local ProductPOVM instances.

[timmintam]

To achieve this we need to be able to construct the explicit tensored operators from a product frame.
It does not scale up but is still very informative to have it for small systems. Moreover, we can explicitly tensor only a part of the product operators, keeping everything efficient (e.g. going from a product of single-qubit operators to a representation of the same frame as a product of two-qubit operators).
The PR leverages the newly introduced "virtual shape" for MultiQubitFrame from #101. It replaces #84 (which will be closed without merging it).

[coveralls]

Pull Request Test Coverage Report for Build 16618538870

Details

• 12 of 45 (26.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.4%) to 97.636%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
povm_toolbox/quantum_info/product_dual.py	3	4	75.0%
povm_toolbox/quantum_info/product_frame.py	8	40	20.0%

Totals
Change from base Build 16263453923:	-2.4%
Covered Lines:	1363
Relevant Lines:	1396

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💛 - Coveralls