prepare grid-based parameterizations

def grid_prep(self):
        """ prepare grid-based parameterizations

        """
        if len(self.grid_props) == 0:
            return

        if self.grid_geostruct is None:
            self.logger.warn("grid_geostruct is None,"\
                  " using ExpVario with contribution=1 and a=(max(delc,delr)*10")
            dist = 10 * float(max(self.m.dis.delr.array.max(),
                                           self.m.dis.delc.array.max()))
            v = pyemu.geostats.ExpVario(contribution=1.0,a=dist)
            self.grid_geostruct = pyemu.geostats.GeoStruct(variograms=v)