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-developments/2_0-Metabolomics-Draft/mzTab_format_specification_2_0-M_draft.adoc#645-isotopomer