Create an index on the fasta file compatible with bowtie2.

def index_bowtie(self):
        """Create an index on the fasta file compatible with bowtie2."""
        # It returns exit code 1 if the fasta is empty #
        assert self
        # Call the bowtie executable #
        sh.bowtie2_build(self.path, self.path)
        return FilePath(self.path + '.1.bt2')