Add sequence to exported GFF3

[kyostiebi]

FASTA is now included in exported GFF3. Please, merge this only after Bug_251_Download_not_working has been merged.

[garrettjstevens]

Has this been tested on exporting very large data? Looking at the code, it looks like it's loading all the sequences into memory, which we want to avoid because we want to keep server memory usage low, and it will be slower. I think for the sequenceStream it will need to use a pipeline like the featureStream does. Just as an outline, that might look like this:

pipeline(
  this.refSeqChunksModel
        .find({ refSeq: refSeqDocs }) // get chunks for all refSeqs in a single query
        .sort({ n: 1 }) // make sure the sort accounts for having all refSeqs
        .cursor(),
  new Transform({
    construct(callback) {
      this.printFasta = true;
      this.printSeqName = true;
      this.remainingLastLine = ''
      callback();
    },
    transform(chunk, encoding, callback) {
      // basically the same logic that's already there, but using `this.push` instead of `sequenceStream.push`
      callback();
    },
    flush(callback) {
      if (this.remainingLastLine) this.push(remainingLastLine)
    },
  }),
  (error) => {
    if (error) {
      this.logger.error('GFF3 export failed')
      this.logger.error(error)
    }
  },
);

The node docs on streams might be helpful here: https://nodejs.org/docs/latest-v18.x/api/stream.html