quad_pix_subm - construct a quad pixel subharmonic method.
construct a quad pixel subharmonic method. it is an experimental method for subharmonic correction for arrays with even axes
subm=quad_pix_subm(3);
Chen jingyuan, Gan guangyong (IAPCM, Beijing,China.)
null_subm , general_subm , lane_subm , johansson_gavel_subm ,