'Only allow saving when all or no head shape elements are set.'

@cached_property
def _get_can_save(self):

    if (not self.stim_chs_ok):
        return False
    has_all_hsp = (np.any(self.dev_head_trans) and np.any(self.hsp) and np.any(self.elp) and np.any(self.fid))
    if has_all_hsp:
        return True
    has_any_hsp = (self.hsp_file or self.fid_file or np.any(self.mrk))
    return (not has_any_hsp)
