RDP nodes under a confidence threshold have to be tag "unclassified"

[loic-couderc]

As mentioned in the issue #54, the RDP confidence score is not propagated to the krona.
But more importantly, the -c (assignment confidence cutoff) has no effect on nodes:

18941|CVKP01000051.3319.4841_m48Prior=0.004400Length=1515NormPrior=0.004222             Root    rootrank        1.0     Bacteria        domain  1.0     Firmicutes      phylum  1.0     Bacilli class   1.0     Lactobacillales order   1.0     Enterococcaceae family  0.41    Melissococcus   genus   0.41

The nodes with confidence < threshold have to be tagged unclassified:

18941|CVKP01000051.3319.4841_m48Prior=0.004400Length=1515NormPrior=0.004222             Root    rootrank        1.0     Bacteria        domain  1.0     Firmicutes      phylum  1.0     Bacilli class   1.0     Lactobacillales order   1.0     unclassified family  0.41    unclassified   genus   0.41```