Allow defined benefits replacement rate to vary by T and J

[jdebacker]

This PR addresses Issue #1243 by allowing the replacement rate in the defined benefits pension system, alpha_db, to vary over time and across lifetime income groups, which allows for reforms of the system in the time path and differential benefits across the population.

Not that that the defined benefits system parameters yr_contrib and avg_earn_num_years are kept as scalars.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.67%. Comparing base (f241c0c) to head (914710b).

Files with missing lines	Patch %	Lines
ogcore/pensions.py	74.07%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1245      +/-   ##
==========================================
+ Coverage   75.49%   75.67%   +0.17%     
==========================================
  Files          24       24              
  Lines        6134     6153      +19     
==========================================
+ Hits         4631     4656      +25     
+ Misses       1503     1497       -6     

Flag	Coverage Δ
unittests	75.67% <75.00%> (+0.17%)	⬆️

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Files with missing lines	Coverage Δ
ogcore/__init__.py	100.00% <100.00%> (ø)
ogcore/parameters.py	81.97% <ø> (ø)
ogcore/pensions.py	72.46% <74.07%> (+3.44%)	⬆️

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[rickecon]

@jdebacker. Thanks for this. Looks great. Merging.