Generate a 2D array of the quasiparticle weight by only selecting the
    first particle data

def pick_flat_z(data):
    """Generate a 2D array of the quasiparticle weight by only selecting the
    first particle data"""
    zmes = []
    for i in data['zeta']:
        zmes.append(i[:, 0])
    return np.asarray(zmes)