code: def install_string(self):
          
          args = [
              "--reference-name", self.reference_name,
              "--annotation-name", self.annotation_name]
          if self.annotation_version:
              args.extend(["--annotation-version", str(self.annotation_version)])
          if self.requires_gtf:
              args.append("--gtf")
              args.append("\"%s\"" % self._gtf_path_or_url)
          if self.requires_protein_fasta:
              args += [
                  "--protein-fasta \"%s\"" %
                  path for path in self._protein_fasta_paths_or_urls]
          if self.requires_transcript_fasta:
              args += [
                  "--transcript-fasta \"%s\"" %
                  path for path in self._transcript_fasta_paths_or_urls]
          return "pyensembl install %s" % " ".join(args)
docs: Add every missing file to the install string shown to the user
          in an error message.