Implement a UnitarySimulator to save the unitary of a quantum circuit

[XYShe]

Add UnitarySimulator backend for computing the unitary transformation corresponding to a quantum circuit

[coveralls]

Pull Request Test Coverage Report for Build 990324626

• 128 of 128 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 970877765:	0.0%
Covered Lines:	7057
Relevant Lines:	7057

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💛 - Coveralls