feat: add rxnNotes to Human-GEM

[haowang-bioinfo]

Description of the issue:

• The rxnNotes field was mentioned in different context, e.g. issues Update EC number field #127 and PRs fix: pmids #254, feat: new function for updating animal GEM #256. And it appears to be a good place for miscellaneous information that are necessary and useful, but without a dedicated field.
• The next relevant question is what information should be stored in this field?

Expected feature/value/output:

• Add rxnNotes field to Human-GEM
• Modify the model manipulate functions (e.g. importYaml, exportYaml) to enable this change

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Done this analysis in the master branch of the repository
• Checked that a similar issue does not exist already

[haowang-bioinfo]

Information that can be stored in the rxnNotes field:

• literature citations not in PMID format
• gap-filled reactions
• spontaneous reactions

[haowang-bioinfo]

Format of the rxnNotes field:

• a field with string elements
• each may have multiple sections that are delimited by semicolon
• keep each element as one line in yml model file (ignore "line too long" warning)