tmva: fix permissions
classify.py: save files in per-session directories
skim: whitespace
classify.py: bugfix saving output to session dir
apply.py: update to new weights dir location
tmva/skim: increase the fraction of sampled events

S/B cut flow:

1. ntuple: raw or after "cleaning"
2. BDT selection: common + sample specific selection
3. trained/tested: half of (2)

| sample | ntuple | BDT selection | trained/tested |
|--------+--------+---------------+----------------|
| Signal | 68k    | 39k           | 20k            |
| Bkg    | 55k    | 10k           | 5k             |

Increase the sampling rate by 4 (39k/10k ~ 4), so that we have similar
order of signal & backround events.