Allow chi_n parameter to vary over the time path

[jdebacker]

This PR updates OG-Core to allow users to specify a $\chi^n_s$ parameter that can vary over the time path (i.e., $\chi^n_{s,t}$).

With this, users can simulate policies that may affect the ability of individuals to work longer at older ages.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (974eac0) 80.38% compared to head (272c969) 80.52%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #897      +/-   ##
==========================================
+ Coverage   80.38%   80.52%   +0.14%     
==========================================
  Files          19       19              
  Lines        4445     4452       +7     
==========================================
+ Hits         3573     3585      +12     
+ Misses        872      867       -5     

Flag	Coverage Δ
unittests	80.52% <94.44%> (+0.14%)	⬆️

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Files	Coverage Δ
ogcore/SS.py	87.58% <ø> (ø)
ogcore/TPI.py	93.02% <100.00%> (ø)
ogcore/utils.py	89.92% <100.00%> (+1.56%)	⬆️
ogcore/parameters.py	84.30% <92.85%> (+0.15%)	⬆️

[rickecon]

@jdebacker. This looks great. Merging.