Ensure library matching uses the mass with the lowest mass error in case of ties

[alex-l-kong]

What is the purpose of this PR?

The _matching_vec helper function for library_matching previously assumed that there would only be one instance of a matching mass, so it took the first one every time. That can no longer be assumed: we will need to find the mass with the lowest mass error relative to ppm since there could be multiple masses.

How did you implement your changes

Implement an additional control flow that iterates through each mass and finds the minimum across all masses in library_peak_df.

Remaining issues

This is inherently a slower implementation since matches will take as long as non-matches. To speed this up, potentially iterate through a pre-sorted library_peak_df by m/z value.

[coveralls]

Pull Request Test Coverage Report for Build 8697236362

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 96.41%

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Totals
Change from base Build 6777233943:	-0.4%
Covered Lines:	157
Relevant Lines:	158

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💛 - Coveralls