Uniformly extract partial structures from a xyz set file.
===========

usage:    ./extract_xyz.sh   N  infile	

N is the interval number of structures that will not be selected.  

Therefore, the larger N, the smaller the size of outfile.

The outfile name is infile-N.xyz 


::

	name=`echo $2|cut -f 1 -d . `
	outfile=$name-$1.xyz
	awk -v sp=$1 '{
		if(a==0){
			if(NF==1) {
				str++;
				num=$1+1;
				if(str%sp==1){
					a=1;
					print $0
				}
				else  a=2;
			}
			else print "err!"
		}
		else if(a==1){
			print $0;
			num--;
			if(num==0) a=0;
		}
		else if(a==2){
			num--;
			if(num==0) a=0;
		}
	} ' $2 > $outfile