Resolve "Allow pickling `DataManager` to speed up re-building of the tree"

This MR adds the DataManager.dump and DataManager.restore methods, which allow storing the data tree as a pickle file and restoring it again.

Details

This MR adjusts parts of the interface such that pickling becomes possible and extends tests that make sure that this works.

Note: Due to an interface change in h5py >= 3.0, this MR restricts the h5py version to < 3.0, see #212 (closed).

Anything to double-check?

Nope.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Not asking for review (time constraints) Reviewed & approved by @

Related issues

Closes #211 (closed)