Save custom groups in h5py data file

[Deepskyhunter]

ARTIQ Pull Request

Description of Changes

Custom attributes stored in dictionary can be passed to Dataset Manager by `save_custom_groups"

Notes:

• If there is dictionary inside the dictionary, it will create a sub-group for it.
• Calling this function second time will replace the previous dictionary

Related Issue #1691

Type of Changes

	Type
	:bug: Bug fix
✓	:sparkles: New feature
	:hammer: Refactoring
	:scroll: Docs

Steps (Choose relevant, delete irrelevant before submitting)

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Code Changes

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• [ ] Add and check docstrings and comments
• [ ] Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

• [ ] Check, test, and update the documentation in doc/. Build documentation (cd doc/manual/; make html) to ensure no errors.

Git Logistics

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Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.

[sbourdeauducq]

Doesn't seem like a well thought-through API.