WIP: Resolve "Add squeeze method to LabelledDataGroup"

This MR adds the LabelledDataGroup.squeeze method, using the same interface as xarray.DataArray.squeeze.

It operates on the underlying member_map, which is a xr.DataArray, and which is used in determining all outside properties of the group.

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
  • Reviewed & approved by @jeremiastraub

Related issues

Closes #158 (closed)