Absolute path to the fasta file with EricScript reference data.

def reference_fasta(self):
        """Absolute path to the fasta file with EricScript reference data."""
        if self._db_location:
            ref_files = glob.glob(os.path.join(self._db_location, "*", self._REF_FASTA))
            if ref_files:
                return ref_files[0]