15N calibration p0@pl0 and cnst21

0. set 1H p1
1. set p0 to 0 ==>  FULL SIGNAL
      zg; qfp; wrp 2 y
2. set p0 to 90 degree ===> NO SIGNAL
      zg; qfp; .md; rep 2

other 15N pulses at cnst22/p21@pl3
for high power calibration change p0/p21 together