fix: do not upload/overwrite context window data

the context data/flags should not be re-uploaded, because it just provide a context for processing the real (unprocessed) data.

The main change is here that we split get_datastream_data into two functions. One to fetch the data, one to fetch the context window. We store the index of the context window within SaQC.attr and before uploading the resulting flags, we remove the stored context-index from the data.