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%

---

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

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9685699622

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 123 of 221 (55.66%) changed or added relevant lines in 3 files are covered.
• 301 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.9%) to 94.982%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rustworkx-core/src/dense_subgraph.rs	79	113	69.91%
src/dense_subgraph.rs	42	106	39.62%

Files with Coverage Reduction	New Missed Lines	%
rustworkx-core/src/dag_algo.rs	5	98.26%
src/json/mod.rs	9	93.23%
src/lib.rs	10	96.81%
src/json/node_link_data.rs	11	87.78%
src/connectivity/mod.rs	16	96.75%
src/digraph.rs	24	98.41%
src/graph.rs	27	97.26%
src/generators.rs	31	94.85%
rustworkx-core/src/generators/hexagonal_lattice_graph.rs	60	66.85%
src/iterators.rs	108	74.78%

Totals
Change from base Build 9565715860:	-0.9%
Covered Lines:	18059
Relevant Lines:	19013

---

💛 - Coveralls