Update interpolation.py

[alanlujan91]

Quick plug in for interpolating lower bound, fixes #971

Please ensure your pull request adheres to the following guidelines:

• 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.

[codecov-io]

Codecov Report

Merging #972 (0ac63d2) into master (449f40f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #972   +/-   ##
=======================================
  Coverage   71.84%   71.85%           
=======================================
  Files          62       62           
  Lines        9204     9205    +1     
=======================================
+ Hits         6613     6614    +1     
  Misses       2591     2591           

Impacted Files	Coverage Δ
HARK/interpolation.py	41.09% <100.00%> (+0.03%)	⬆️

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