code: def get_chrs_from_bam(self, file_name):
          
          x = subprocess.check_output(self.tools.samtools + " view -H " + file_name + " | grep '^@SQ' | cut -f2| sed s'/SN://'", shell=True)
          # Chromosomes will be separated by newlines; split into list to return
          return x.split()
docs: Uses samtools to grab the chromosomes from the header that are contained
          in this bam file.