Implement set_value_as_xarray

[Peter9192]

Made a start, but there's some bumps in the road.

        # TODO: what if data is NaN? Model-specific back-conversion? Don't convert to NaN in getters?
        # TODO: wflow not settable (rank mismatch, should be 2???!)
        # TODO: pcrglob discharge not settable (but works for temperature)
        # TODO: add tests like so:
        """
        name = 'temperature'
        da = model.get_value_as_xarray(name)
        orig = model.get_value(name)
        model.set_value_as_xarray(name, da)
        new = model.get_value(name)
        assert all(orig == new)
        """

Closes #182

[sonarcloud]

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