calculates and stores the set of indices
        ix=[0, nx-1], iy=[0, ny-1] for data of shape (nx, ny)

def calc_indices(self, shape):
        """calculates and stores the set of indices
        ix=[0, nx-1], iy=[0, ny-1] for data of shape (nx, ny)"""
        if len(shape) == 2:
            ny, nx = shape
        elif len(shape) == 3:
            ny, nx, nchan = shape

        inds = []
        for iy in range(ny):
            inds.extend([(ix, iy) for ix in range(nx)])
        self.conf.indices = np.array(inds)