Hexagonal graph with consecutive node indices

[georgios-ts]

Closes #373.

This commit modifies hexagonal_lattice_graph and directed_hexagonal_lattice_graph to create graphs with consecutive node indices. This is possible with minor changes in the logic of the original source code, roughly we should shift the node indexes by one (or two in the last column) except the first column that we don't need a shift.

This is important if we ever use these functions to generate a CouplingMap in terra where the underlying graph assumes consecutive node indices.

[coveralls]

Pull Request Test Coverage Report for Build 1068262658

• 98 of 102 (96.08%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 96.215%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/generators.rs	98	102	96.08%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	1	97.72%

Totals
Change from base Build 1060816472:	-0.004%
Covered Lines:	8491
Relevant Lines:	8825

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💛 - Coveralls

[mtreinish]

Awesome, thanks for fixing this! Everything LGTM and I tested it locally for a bunch of inputs and it all looked good.