Fixed bug in demux reducer to allow for [6,12]-base barcodes

The current version of this code fetches a sequence and checks whether its length is 7 (hard-coded).
If the length is 7, then the last base is discarded and the first 6 are kept (hard-coded too).

These lines should be modified to allow for sequences of length in interval [7,13], i.e. BAR_CODE_{MIN,MAX}_LENGTH + 1, defined in https://github.com/crs4/seal/blob/master/src/it/crs4/seal/demux/SampleSheet.java