_compressFile(src, dest, callback) {
    fs.access(src, fs.F_OK, (err) => {
      if (err) {
        return callback();
      }
      var gzip = zlib.createGzip();
      var inp = fs.createReadStream(src);
      var out = fs.createWriteStream(dest);
      out.on('finish', () => {
        fs.unlink(src, callback);
      });
      inp.pipe(gzip).pipe(out);
    });
  }
Compresses src to dest with gzip and unlinks src
@param {string} src - path to source file.
@param {string} dest - path to zipped destination file.
@param {Function} callback - callback called after file has been compressed.
@returns {undefined}
@private