Corrected and added the indicators to determine the chemical rank

[sshojiro]

Formerly, I have added indicators that can determine the chemical rank based on spectral data. However, I have found mistakes in both the original definition in the reference and my implementation. I corrected the IND indicator, following another reference[1].

Along with the correction of the implementation, I have added some documentation on the methods and test code. Please check the compatibility with your repository.

Thanks in advance!

[1] "An automated procedure to predict the number of components in spectroscopic data"

[codecov[bot]]

Codecov Report

Merging #8 into 0.3.X will increase coverage by 0.73%. The diff coverage is 97.56%.

@@            Coverage Diff             @@
##            0.3.X       #8      +/-   ##
==========================================
+ Coverage   97.58%   98.32%   +0.73%     
==========================================
  Files          13       13              
  Lines        1407     1429      +22     
==========================================
+ Hits         1373     1405      +32     
+ Misses         34       24      -10

Impacted Files	Coverage Δ
pymcr/tests/test_rank.py	100% <100%> (ø)	:arrow_up:
pymcr/condition.py	100% <100%> (ø)	:arrow_up:
pymcr/rank.py	92.68% <96.15%> (+33.3%)	:arrow_up:

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