Raise NotImplementedError when using extrapolation

[freemansw1]

Resolves the first part of #165 by raising a NotImplementedError when users try to use extrapolation.

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[codecov]

Codecov Report

Merging #177 (be85f36) into RC_v1.4.0 (955e799) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           RC_v1.4.0     #177      +/-   ##
=============================================
+ Coverage      34.93%   35.00%   +0.06%     
=============================================
  Files             11       11              
  Lines           2098     2100       +2     
=============================================
+ Hits             733      735       +2     
  Misses          1365     1365              

Flag	Coverage Δ
unittests	35.00% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tobac/tracking.py	65.13% <100.00%> (+0.65%)	⬆️

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[JuliaKukulies]

This looks all good to me - approved!

[kelcyno]

This works for me, glad to approve.

[freemansw1]

Given the small nature of the changes and the two approving reviews, I am merging this. Thanks for the quick reviews, @kelcyno and @JuliaKukulies!