Implement lazy data saving to xarray output.

[pp-mo]

A feature extension to keep data lazy in xarray output

[pp-mo]

NOTE: thinking this over, I now think this mechanism is pretty awful.

If we simply accept that we can only replace lazy content wholesale, then we can provide a method specifically for that.
E.G.

    def set_lazy_content(self, array):
        if not all(hasattr(array,x) for x in ('compute', '__setitem__', 'shape')):
            raise ValueError(f'Assigned content, {array} must be a lazy array.')
        self._xr = array

That way,

1. the existence of set_lazy_content can be used as the capability test used by the assigner ('da.store' replacement)
2. we don't need the complex _indexes_address_full_shape code.