[fluent] Change container for nodes to xr.DataTree

[jinmannwong]

Description

Change to use xr.DataTree instead of xr.DataArray for containing nodes to support surface and pressure levels.

• Fluent API has been extended to add path argument, which specifies the branch of the DataTree to act on and methods added to split existing single node array into separate arrays along different branches of tree
• Node arrays are only allowed at the leaves of the tree i.e. the tree must be hollow

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.