Resolve "Allow non-`time`-labelled nodes and edges in `NetworkGroup`"

What does this MR do?

This MR ...

Can this MR be accepted?

• Implemented the changes
• Tests added or adjusted
• Pipeline passes
• Version bumped to 0.8.1 (in dantro/__init__.py)
• Changelog entry added
• Approved by @blsqr

Anything to double-check?

Related issues

Closes #89 (closed)