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 fix
✓	✨ New feature
	🔨 Refactoring
	📜 Docs

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Licensing

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[sbourdeauducq]

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