Add isotopomer validation rule to mapping file

[nilshoffmann]

Isotopomer is currently not validated in the mapping file.
If the element is not-null, it MUST be the exact term MS:1002957, only varying values are allowed.
This would boil down to an optional rule, meaning that if it is present, it will be validated but no error will be raised if it is null.

https://github.com/HUPO-PSI/mzTab/blob/master/specification_document-releases/2_0-Metabolomics-Release/mzTab_format_specification_2_0-M_release.adoc#645-isotopomer

[philouail]

I know this is an old issue, but please go through the different steps of the template below, to make it easy for us to understand what to do next with this:

Is your feature request related to a problem? Please describe.
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Submitter Tasks

• Provide a pull request (Please add the link)
• Schema: Update mzTab_m_openapi.yml (View File)
• Specification: Update mzTab_format_specification_2_1-M.adoc with an example (View File)
• Example files: if possible, modify an existing or add a new example file (Existing example files)

Maintainer & WG Tasks

• Triage: Assign labels and Milestones to this issue.
• Discuss and review and approve PR.

Next Steps, once we reach an agreement on the changes above:

• Create a downstream issue in the jmzTab-m repo (https://github.com/lifs-tools/jmzTab-m/issues)
• Implement and test changes in jmzTab-M
• Implement changes in R & Python implementations