Create make_Gmax_v20_chroms_integers.pl

Convert sequence headers for Ensembl Gmax v1.0.20 (ftp://ftp.ensemblgenomes.org/pub/plants/release-20/fasta/glycine_max/dna/Glycine_max.V1.0.20.dna.toplevel.fa.gz) to simple numeric ones compatible with the Tassel 3.0 GBS pipeline.