[FEATURE] add python compatible get precursor spectrum

[timosachsenberg]

Description

Add helper for pyopenms to retrieve the index of the precursor spectrum

Checklist:

• Make sure that you are listed in the AUTHORS file
• Add relevant changes and new features to the CHANGELOG file
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• Updated or added python bindings for changed or new classes. (Tick if no updates were necessary.)

How can I get additional information on failed tests during CI:

If your PR is failing you can check out

• http://cdash.openms.de/index.php?project=OpenMS and look for your PR. If you click in the column that lists the failed tests you will get detailed error messages.

Note:

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[timosachsenberg]

rebuild jenkins

[timosachsenberg]

tests passed

[oliveralka]

IMHO Looks good!

[axelwalter]

very useful, thanks!