use REPL doctest for center/centered

[johnnychen94]

Resolves #253 (comment)

Although the script mode doctest is shorter, it somehow doesn't look very nice. This PR switches back to the REPL mode doctest for center/centered.

[codecov]

Codecov Report

Merging #254 (ca63d80) into master (6cbc7f1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #254   +/-   ##
=======================================
  Coverage   95.41%   95.41%           
=======================================
  Files           5        5           
  Lines         436      436           
=======================================
  Hits          416      416           
  Misses         20       20           

Impacted Files	Coverage Δ
src/OffsetArrays.jl	98.29% <ø> (ø)
src/axes.jl	100.00% <0.00%> (ø)

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[jishnub]

Could you also give an example of what cp is supposed to do in centered? Otherwise looks good to me.

[johnnychen94]

Regardless of the implementation simplicity, I have to admit this became a bad design practice. See #256 for more comments.

It's unlikely to correct it without breaking change, so for now, I'll just assume the current behavior is wanted and add some docstring to it.