function made to compute the amplitude given a windows around a phase

Input:
trace: data array
window: time window of size 2, matalab serial datenum
t_begin: time of beginning of the trace, matlab serial datenum
rsample: sampling rate
units: nothing or ‘nm’
flag_plot; 0 or 1

Output:
AMP: structure containing period, max pk2pk/2 amplitude and
arrival (matlab datenum)