Update default calibration

[jdebacker]

This PR updates the default calibration for the OG-USA model. Changes include:

1. A fix to macro_params.py, which corrects double counting of social security payments when determining the $\alpha_G$ parameter. As a result, $\alpha_G$ is significantly larger.
2. Changes default start year to 2025
3. Updates AIME breakpoints for 2024 values
4. Rounds numbers to 2-4 digits, depending on the parameter.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.22%. Comparing base (42a4ff8) to head (a973a1f).
Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #107   +/-   ##
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  Coverage   45.22%   45.22%           
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  Files          18       18           
  Lines        1404     1404           
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  Hits          635      635           
  Misses        769      769           

Flag	Coverage Δ
unittests	45.22% <100.00%> (ø)

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Files with missing lines	Coverage Δ
ogusa/__init__.py	100.00% <100.00%> (ø)
ogusa/macro_params.py	100.00% <ø> (ø)

[rickecon]

@jdebacker. I just have the one question about updating values in the ogusa_default_parameters.json instead of updating them in the sample run script. The current status of this PR is good to go, but I think I would prefer moving those changes to the ogusa_default_parameters.json file. Let me know what you think, and I'll merge this.

[jdebacker]

@rickecon Thanks for the review. I just made the suggested changes.

[rickecon]

@jdebacker. This looks great. Merging.