Add function to find densest subgraph

[mtreinish]

This commit adds a new function find_densest_subgraph_of_size() to
retworkx. This function is used to find a subgraph in a given graph of a
specified size that has the highest degree of connectivity of nodes.

Fixes #570

TODO:

• Add more tests
• Test with qiskit-terra's dense layout

[coveralls]

Pull Request Test Coverage Report for Build 2578936521

• 97 of 131 (74.05%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 96.926%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dense_subgraph.rs	95	129	73.64%

Files with Coverage Reduction	New Missed Lines	%
retworkx-core/src/centrality.rs	3	98.95%
retworkx-core/src/max_weight_matching.rs	4	99.34%

Totals
Change from base Build 2494694599:	-0.2%
Covered Lines:	12772
Relevant Lines:	13177

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3093343024

• 98 of 137 (71.53%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 96.851%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dense_subgraph.rs	96	135	71.11%

Totals
Change from base Build 3090511963:	-0.3%
Covered Lines:	12731
Relevant Lines:	13145

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💛 - Coveralls