Update macro parameters

[jdebacker]

This PR adds to macro_params.py to draw from IMF data for the $\alpha_T$ and $\alpha_G$ parameters.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.46%. Comparing base (e8e0fcb) to head (8060f56).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   57.06%   56.46%   -0.61%     
==========================================
  Files          12       12              
  Lines         361      356       -5     
==========================================
- Hits          206      201       -5     
  Misses        155      155              

Flag	Coverage Δ
unittests	56.46% <100.00%> (-0.61%)	⬇️

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[jdebacker]

This PR is ready for review and merge.