Refactor setup to support modular requirements lists #42

[a-r-j]

Reference Issues/PRs

42

What does this implement/fix? Explain your changes

Refactors dependencies into blocks that can be switched out.

Adds option for install commands like:

pip install gauche[cpu]
pip install gauche[cu116]
pip install gauche[117]
pip install gauche[dev]

What testing did you do to verify the changes in this PR?

No testing. Please verify before merging.

Pull Request Checklist

• [ ] Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• [ ] Added appropriate unit test functions in the ./gauche/tests/* directories (if applicable)
• [ ] Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• [ ] Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/kernels/test_graph_kernels.py)
• [ ] Checked for style issues by running black . and isort .