Compute Gamma Index of Spatial Autocorrelation for GeoRaster

        Usage:
        geo.pysal_Gamma(permutations = 1000, rook=True, operation='c')

        arguments passed to raster_weights() and pysal.Gamma
        See help(gr.raster_weights), help(pysal.Gamma) for options

def pysal_Gamma(self, **kwargs):
        """
        Compute Gamma Index of Spatial Autocorrelation for GeoRaster

        Usage:
        geo.pysal_Gamma(permutations = 1000, rook=True, operation='c')

        arguments passed to raster_weights() and pysal.Gamma
        See help(gr.raster_weights), help(pysal.Gamma) for options
        """
        if self.weights is None:
            self.raster_weights(**kwargs)
        rasterf = self.raster.flatten()
        rasterf = rasterf[rasterf.mask==False]
        self.Gamma = pysal.Gamma(rasterf, self.weights, **kwargs)