Using createStratumInstance with traits which have nested objectTraits

[nf-s]

We can't createStratumInstance with traits which have nested objectTraits (or arrays...)

From slack:
... in WMS catalog item we have LegendTraits and WebMapServiceAvailableLayerStylesTraits which also includes LegendTraits

We copy LegendTraits from availableStyles in @computed get legends()... - https://github.com/TerriaJS/terriajs/blob/next/lib/Models/WebMapServiceCatalogItem.ts#L163-L166

if (layerStyle !== undefined && layerStyle.legend !== undefined) {
  result.push(
    <StratumFromTraits<LegendTraits>>(<unknown>layerStyle.legend)
  );
}

Is there a nice way of copying all traits across instead of having to use createStratumInstance for each LegendTraits (and then createStratumInstance for each LegendItemTraits )?

For example

const legend = createStratumInstance(LegendTraits, layerStyle.legend)
legend.url = 'change the url to something...'
result.push(legend)

[wibily]

Not needed/wrong