Add API to attach hdf5 attributes to datasets

[yuk-m-labs]

ARTIQ Pull Request

Description of Changes

Expose API to let user attach HDF5 attributes to datasets. The attributes can be attached to the HDF5 group datasets, or to HDF5 datasets under this group by specifying the key.

Usage demo is given in examples/no_hardware/repository/hdf5_attributes.py

Related Issue

Closes #1691

Type of Changes

	Type
✓	✨ New feature
✓	📜 Docs

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• Use correct spelling and grammar.
• Update RELEASE_NOTES.rst if there are noteworthy changes, especially if there are changes to existing APIs.
• Close/update issues.
• Check the copyright situation of your changes and sign off your patches (git commit --signoff, see copyright).

Code Changes

• Run flake8 to check code style (follow PEP-8 style). flake8 has issues with parsing Migen/gateware code, ignore as necessary.
• Test your changes or have someone test them. Mention what was tested and how.
• Add and check docstrings and comments
• Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Git Logistics

• Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.
• Write short & meaningful commit messages. Review each commit for messages (git show). Format:

  topic: description. < 50 characters total.
  
  Longer description. < 70 characters per line
  

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]

This example should become a unit test and the comments assertions.

[thomasfire]

Moved to #1997