Save model or subset of model as friendly_data

[sjpfenninger]

Problem description

As part of removing visualisation and cleaning up the core code, we want to make it easier to save all or some results so that they can be directly plugged into a separate visualisation tool. friendly_data is probably the way we want to do this.

[suvayu]

xarray.Dataset to friendly data package and vice versa is now supported.

• Issue: Multiple analysis backends sentinel-energy/friendly_data#6 (comment)
• code: see to_da, to_dst, to_mfdst, & from_dst in friendly_data.converters
• API docs