async getImageCount() {
    let index = 0;
    // loop until we run out of IFDs
    let hasNext = true;
    while (hasNext) {
      try {
        await this.requestIFD(index);
        ++index;
      } catch (e) {
        if (e instanceof GeoTIFFImageIndexError) {
          hasNext = false;
        } else {
          throw e;
        }
      }
    }
    return index;
  }
Returns the count of the internal subfiles.

@returns {Promise<number>} the number of internal subfile images