Create contigs_statistics.py

This program is based on contig_stats.pl. It takes one fasta file (presumably obtained from a metagenome assembler) and one threshold value as input and calculates contig or scaffold metrics. Assembly capacity metrics such as number of contigs, total number of bases,  the  average size, the maximum contigh length. It also calculates some stats of contiguity such as the N50 values or contigs in N50.