Create a slice object according to the ppm scale

    Parameters
    ----------
    f_ppm : float array
        The frequency bins (in the ppm scale). Assumed to be descending

    lb,ub : float
        The lower/upper bounds for indexing
        
    Returns
    -------
    idx : slice object which can be used to index

def ppm_idx(f_ppm, lb, ub):
    """
    Create a slice object according to the ppm scale

    Parameters
    ----------
    f_ppm : float array
        The frequency bins (in the ppm scale). Assumed to be descending

    lb,ub : float
        The lower/upper bounds for indexing
        
    Returns
    -------
    idx : slice object which can be used to index 
    """
    idx0 = np.argmin(np.abs(f_ppm - lb))
    idx1 = np.argmin(np.abs(f_ppm - ub))
    return slice(idx1, idx0)