DicomStreamReader can now read only the file meta info and no more

* DicomStreamReader now provides an optional stopping condition based on the
  File Meta Info Group Length (0002,0000). This tag is Type 1, thus it should
  be able to stop the reading before it consumes the first 4 bytes of a
  deflated data set.
* The StorePixelDataReferences read option is now ignored if the stream is not
  a FileStream.

Fixes #12