remove unnecessary use of interpolation

[JimPrudent]

interp1d is by default a linear interpolation

https://github.com/scipy/scipy/blob/v1.7.0/scipy/interpolate/interpolate.py#L332-L721

[arvkevi]

Thanks for the PR @JimPrudent.
My Travis CI was outdated, so the unit tests weren't run in the Travis environment. I think I've fixed the issue. I checked out your branch locally and all the unit tests passed.

Can you add an empty commit to trigger the build, something like:
git commit --allow-empty -m "Trigger Build"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff            @@
##             main       #97   +/-   ##
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  Coverage        ?   100.00%           
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  Files           ?         5           
  Lines           ?       210           
  Branches        ?         0           
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  Hits            ?       210           
  Misses          ?         0           
  Partials        ?         0           

Flag	Coverage Δ
unittests	100.00% <100.00%> (?)

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Files	Coverage Δ
kneed/knee_locator.py	100.00% <100.00%> (ø)

[arvkevi]

This was failing CI and interp1d is legacy now.