code: public IonReader build(byte[] ionData, int offset, int length)
      {
          return makeReader(validateCatalog(), ionData, offset, length);
      }
docs: Based on the builder's configuration properties, creates a new IonReader
  instance over the given block of Ion data, detecting whether it's text or
  binary data.
  <p>
  This method will auto-detect and uncompress GZIPped Ion data.
  
  @param ionData the source of the Ion data, which is used only within the
  range of bytes starting at {@code offset} for {@code len} bytes.
  The data in that range may be either Ion binary data or UTF-8 Ion text.
  The reader retains a reference to the array, so its data must not be
  modified while the reader is active. Must not be null.
  @param offset must be non-negative and less than {@code ionData.length}.
  @param length must be non-negative and {@code offset+length} must not
  exceed {@code ionData.length}.
  
  @see IonSystem#newReader(byte[], int, int)