adding get_query_function

[cedricboudinet]

Hello
Here is a PR adding a function enabling one to read the modbus function of a query.
It can be useful for modbus server development.
If you are ok with it I can complete the docs as well.
Bests
Cedric

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[mhei]

Usually open source development works that way, that you create a full PR, i.e. including documentation etc.
In your documentation, I would for example expect to find a description of your use case for this new function - at the moment, you only described it roughly.

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[cla-bot]

We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, please fill https://forms.gle/5635zjphDo5JEJQSA to get added. Your document will be manually checked by the maintainer. Be patient...