Echo: optional phase sweep of second pi/2 pulse
Echo: optional phase sweep of second pi/2 pulse

if periods=N>0, sweeps the phase of the second pi/2 pulse to obtain an
artificial detuning in the echo measurment, with N periods
--DR

Plourde-Research-Lab/PyQLab,calebjordan/PyQLab,BBN-Q/PyQLab,rmcgurrin/PyQLab