[Minor API] Add to_xarray API to State

[FlorianDeconinck]

Description

Expose a to_xarray re-formatting a State to a xr.DataTree by reusing code written for to_netcdf

This is preliminary to a wider "State Recorder" idea being explored elsewhere.

How has this been tested?

Simple exposing of existing API

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. add new modules to docs/docstrings/)
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[twicki]

Looks good, thanks!

[romanc]

👍