Add unit support for to/from xarray

[znicholls]

Is your feature request related to a problem? Please describe.

xarray has support for units via pint too. We don't support this currently.

Describe the solution you'd like

Add support for getting xarray objects including units to scmdata and to parse xarray objects with units.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.