Residual use in Tree

[HGuillemet]

In the forward method of the Tree class in dla.py, the residual argument is always ignored (overriden line 209). Is it what is wanted ? If it is, then building project in the constructor and calling its forward method it line 209 is useless when levels > 1. Some computation could be saved. Also bottom computation is not used if levels > 1 and level_root is false.