def fermion_avg(efermi, norm_hopping, func):
    """calcules for every slave it's average over the desired observable"""
    if func == 'ekin':
        func = bethe_ekin_zeroT
    elif func == 'ocupation':
        func = bethe_filling_zeroT

    return np.asarray([func(ef, tz) for ef, tz in zip(efermi, norm_hopping)])
calcules for every slave it's average over the desired observable