function made to output the list of stations to be ouputed for amplitude
calculation, this is a very simple function but quite useful as it can be used
a lot of times

Input:
mainfile: main parameter file for PSPicker (string)

Output:
stations_amp: stations that will be used for amplitude calculation (cellarray)

Example:
stations_amp=ampstations_PARAM(‘mainfile_Gorkha.txt’)