Allows for age-varying interest rate in ConsumptionSaving models

[alanlujan91]

This PR allows for interest rate (Rfree) to be "age"-varying in ConsumptionSaving models.

Currently it is only tested in ConsIndShockModel, but it should work on all other models by design.

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[alanlujan91]

Requesting review from @wdu9. Let me know if this works as you expect!

[codecov-commenter]

Codecov Report

Merging #1150 (491f0d5) into master (0171e0d) will increase coverage by 0.00%.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##           master    #1150   +/-   ##
=======================================
  Coverage   74.06%   74.07%           
=======================================
  Files          70       70           
  Lines       10807    10838   +31     
=======================================
+ Hits         8004     8028   +24     
- Misses       2803     2810    +7     

Impacted Files	Coverage Δ
...nsumptionSaving/tests/test_IndShockConsumerType.py	75.86% <57.14%> (+0.78%)	⬆️
HARK/ConsumptionSaving/ConsIndShockModel.py	85.40% <85.71%> (-0.46%)	⬇️

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[wdu9]

Requesting review from @wdu9. Let me know if this works as you expect!

Looks great! Thanks!