@ValidateLarchPlugin
def f0_ions(element=None, _larch=None):
    xdb = get_xraydb(_larch)
    return xdb.f0_ions(element=element)
return list of ion names supported in the f0() calculation from
dataset/ETHPy150Open xraypy/xraylarch/plugins/xray/xraydb_plugin.py/f0_ions