exclude ambiguous characters in kmer
exclude ambiguous characters in kmer

MG-RAST/kmerspectrumanalyzer,MG-RAST/kmerspectrumanalyzer,wltrimbl/kmerspectrumanalyzer,MG-RAST/kmerspectrumanalyzer,wltrimbl/kmerspectrumanalyzer,wltrimbl/kmerspectrumanalyzer