code: def _throat_c2c(self):
          r
          net = self.network
          Nt = net.num_throats()
          p_cen = net['pore.centroid']
          t_cen = net['throat.centroid']
          conns = net['throat.conns']
          p1 = conns[:, 0]
          p2 = conns[:, 1]
          v1 = t_cen-p_cen[p1]
          v2 = t_cen-p_cen[p2]
          check_nan = ~sp.any(sp.isnan(v1 + v2), axis=1)
          value = sp.ones([Nt, 3], dtype=float)*sp.nan
          for i in range(Nt):
              if check_nan[i]:
                  value[i, 0] = sp.linalg.norm(v1[i]) - self.network.fiber_rad
                  value[i, 1] = self.network.fiber_rad*2
                  value[i, 2] = sp.linalg.norm(v2[i]) - self.network.fiber_rad
          return value[net.throats(self.name)]
docs: r"""
          Calculate the center to center distance from centroid of pore1 to
          centroid of throat to centroid of pore2.