Fix method: margin for balancing

[dhondta]

Reference: fix method, balance function
Goal: Fix balancing of labels, taking a margin into account (e.g. 20%, that is if 2 labels then [40,60]%)
Points of attention:

• Take NOT_LABELLED into account (cfr unsupervised or semi-supervised learning)
• Adapt way to compute balance metric in quality scoring ; will only handle packed/not-packed as of Aug 25