Fix documentation for extrapolate

[mkitti]

Fix #580 .

Need to setup jldoctests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.70%. Comparing base (c66a77e) to head (4ad3a7b).
Report is 47 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   87.75%   87.70%   -0.05%     
==========================================
  Files          28       28              
  Lines        1886     1887       +1     
==========================================
  Hits         1655     1655              
- Misses        231      232       +1     

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