Add functions for shifting age related parameters, computing pct changes in output

[jdebacker]

This PR adds two new utilities to the utils.py modules:

1. shift_bio_clock: this function can shift age-related parameters that also vary over time (e.g., mortality rates) by a certain number of years, with a transition over time
2. pct_change_unstationarized: this function computes percentage changes in unstationarized output (useful if the baseline and reform runs feature different rates of economic growth or population growth).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.94737% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 73.17%. Comparing base (3d638e8) to head (637e0c8).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #915      +/-   ##
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- Coverage   79.84%   73.17%   -6.68%     
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  Files          19       19              
  Lines        4506     4563      +57     
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- Hits         3598     3339     -259     
- Misses        908     1224     +316     

Flag	Coverage Δ
unittests	73.17% <78.94%> (-6.68%)	⬇️

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Files	Coverage Δ
ogcore/utils.py	88.47% <78.94%> (-1.45%)	⬇️

... and 5 files with indirect coverage changes