code: def _get_facet_and_simplex(self, comp):
          
          c = self.pd_coords(comp)
          for f, s in zip(self.facets, self.simplexes):
              if s.in_simplex(c, PhaseDiagram.numerical_tol / 10):
                  return f, s
          raise RuntimeError("No facet found for comp = {}".format(comp))
docs: Get any facet that a composition falls into. Cached so successive
          calls at same composition are fast.