rename labels for performance increase #38
Description
MurTree internally uses a vector of instances for each possible label. This creates issues, for example, when the two possible labels are 1000 and 2000. MurTree now assumes there are 2001 unique labels, which significantly impacts performance.
Therefore, labels should be renamed to the 0...num_labels range before being passed to the c++ solver.
Preferably, the transformation is stored and the original labels are used when exporting the tree with export_tree or export_dot