Reflect input/output whitepoints in `whitebalance`

[kimikage]

This is a preparation for supporting the Bradford transform. (cf. #278) This also updates the document for whitebalance.

[codecov[bot]]

Codecov Report

Merging #507 (297aa43) into master (f71d7a1) will increase coverage by 0.56%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #507      +/-   ##
==========================================
+ Coverage   93.84%   94.41%   +0.56%     
==========================================
  Files           9        9              
  Lines        1154     1163       +9     
==========================================
+ Hits         1083     1098      +15     
+ Misses         71       65       -6     

Impacted Files	Coverage Δ
src/algorithms.jl	69.29% <100.00%> (+8.34%)	:arrow_up:

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[kimikage]

I may rewrite some of the internal functions for bradford_transform. For this reason, I made this a draft PR, but will not make any major changes. I will merge this after I submit the PR for bradford_transform.