function made to read a station file containing poles and zeros
the input file must be:

station channel comp gain poles zeros
AVA : SH Z 5 0,0,0 2,1,3
RES : SH Z 5 0,0,0 2,1,3
AVA SL Z 5
GKN SH Z 0.11650840E+07 0.13180000E+02-i0.25020000E+02

It’s possible that gains and zeros are not defined, so just leave the last
columns empty

Output:
D : structure containing Gain, Poles and Zeros

file=’/Users/baillard/_Moi/Projets/Nepal/Picking/stations_PZ.txt’;