Fast pruning

FastPruning is a patch of the algorithm pruning the tree down to a target number of bins. 
The previous implementation physically removed a bin at once (redefining the links for all of the other bins each time), the current implementation marks down all of the bin that need to be removed and proceed with the removal.
Another bug was found and fixed: the "minimal pdf" used in the denominator was actually the min "id" of the two bins which doesn't make sense at all and it is surprising was never reported before. 

See merge request !4