add GraphData subgraph, shortest length

[tonydavis629]

Description

Adds in a graphdata.subgraph method to extract subgraphs from a graphdata object, and a shortest path length utility function to find the shortest distances of a root node to nodes at a specified maximum distance.

Type of change

Please check the option that is related to your PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
  • In this case, we recommend to discuss your modification on GitHub issues before creating the PR
• Documentations (modification for documents)

Checklist

• My code follows the style guidelines of this project
• Run yapf -i <modified file> and check no errors (yapf version must be 0.32.0)
• Run mypy -p deepchem and check no errors
• Run flake8 <modified file> --count and check no errors
• Run python -m doctest <modified file> and check no errors
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

[rbharath]

Looks good, but we have some yapf errors. Feel free to merge in once those are addressed

[tonydavis629]

Maybe one day I'll understand why my yapf 0.32.0 is different from the CI's yapf 0.32.0, but not today.