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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _post_read_flds(flds, header):
 """Process flds to handle sphericity."""

    if flds.shape[0] >= 3 and header['rcmb'] > 0:
        # spherical vector
        header['p_mesh'] = np.roll(
            np.arctan2(header['y_mesh'], header['x_mesh']), -1, 1)
        for ibk in range(header['ntb']):
            flds[..., ibk] = _to_spherical(flds[..., ibk], header)
        header['p_mesh'] = np.roll(
            np.arctan2(header['y_mesh'], -header['x_mesh']) + np.pi, -1, 1)
    return flds