Add interpolate functions, but don't remove interp.

[rofinn]

Closes #28

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   95.63%   95.63%           
=======================================
  Files          10       10           
  Lines         206      206           
=======================================
  Hits          197      197           
  Misses          9        9

Impacted Files	Coverage Δ
src/Impute.jl	73.68% <ø> (ø)	⬆️

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