Update some scripts to make them work.

* Moved a function from utils to apply.py because numpy didn't load.
* Added all the combined variables to extend.cc (under different names, to be compatible with the variable names of the old BDT).
* Changed apply.py to use those variables. 
* Changed TCut + TCut to a declaration and a += call; somehow the former doesn't work, while the latter does.
* Removed the use of TTree::SetBranchAddress (doesn't work in Python).