Add leakage parameter for government infrastructure investment

[MahadevBalla]

Closes #1086

Summary

• Add new parameter infra_investment_leakage_rate to default_parameters.json
• Apply leakage to public capital accumulation in fiscal.get_K_g()
• Update the get_K_g() docstring to reflect the new law of motion
• Add tests for the no-leakage baseline and a positive-leakage SS case

Testing

• pytest tests/test_fiscal.py
• pytest tests/test_fiscal.py -k "get_K_g"

Output / Validation

pytest tests/test_fiscal.py -k "get_K_g"
3 passed, 23 deselected in 4.38s

pytest tests/test_fiscal.py
26 passed in 8.77s

[jdebacker]

Let's call this phi_g as in the doctoring.

[jdebacker]

@MahadevBalla Thanks for the PR!

I left one comment above. In addition, please run make format from the top level directory to format the .py files.

I would be good to update the theory docs in .docs/book/content/theory to reflect this new parameter. But we can do that in another PR if that's a problem.

I will try to complete and merge PR #1097 soon -- once those changes are in this branch, the test failures we're seeing here should be resolved.

[MahadevBalla]

Thanks for the review @jdebacker! I’ve made the phi_g change and run make format.
I won’t be able to work on the theory docs update until Sunday, but I can follow up with that separately after that.