Provide option to assume zero initial state in ansatz generation

[garrison]

The ansatz begins with ZXZ decompositions, but the first RZGate on each qubit has no effect if acting on the initial state. So, this adds an option to remove that parameter on each qubit.

It's left as an option since it does not make sure to do in the case of unitary AQC.

Right now the option is called assume_initial_zero_state. I'm definitely open to discussing the name of the option.

TODO

• add new argument to docstring - will do this in follow-up PR
• use this flag in the tutorial and tests

[coveralls]

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Details

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 95.39%

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Totals
Change from base Build 11638757861:	0.4%
Covered Lines:	600
Relevant Lines:	629

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💛 - Coveralls

[garrison]

@jakelishman has informed me of Qiskit/qiskit#12911, which used the name qubits_initially_zero for this concept. I like that name better and will update this PR shortly.