Returns a label (str) for this particular slab based
            on composition, coverage and Miller index.

def create_slab_label(self):
        """
        Returns a label (str) for this particular slab based
            on composition, coverage and Miller index.
        """

        if "label" in self.data.keys():
            return self.data["label"]

        label = str(self.miller_index)
        ads_strs = list(self.ads_entries_dict.keys())

        cleaned = self.cleaned_up_slab
        label += " %s" % (cleaned.composition.reduced_composition)

        if self.adsorbates:
            for ads in ads_strs:
                label += r"+%s" % (ads)
            label += r", %.3f ML" % (self.get_monolayer)
        return label