Concatenates an NdOverlay of Image types into a single 3D
        xarray Dataset.

def concatenate(cls, overlay):
        """
        Concatenates an NdOverlay of Image types into a single 3D
        xarray Dataset.
        """
        if not isinstance(overlay, NdOverlay):
            raise ValueError('Only NdOverlays can be concatenated')
        xarr = xr.concat([v.data.transpose() for v in overlay.values()],
                         pd.Index(overlay.keys(), name=overlay.kdims[0].name))
        params = dict(get_param_values(overlay.last),
                      vdims=overlay.last.vdims,
                      kdims=overlay.kdims+overlay.last.kdims)
        return Dataset(xarr.transpose(), datatype=['xarray'], **params)