zoomTo should not be disabled for CesiumTerrainCatalogItem

[thhomas]

I have a use case where I have a terrain layer converted from a local DEM that I would like to visualise.
With the current behaviour, I cannot specify a ideal zoom thus the user has to zoom in to the extent where the layer is available which is not very handy.

[thhomas]

terriajs/lib/Models/Catalog/CatalogItems/CesiumTerrainCatalogItem.ts

Lines 37 to 40 in 0a3be8c

	@override
	get disableZoomTo() {
	return true;
	}