tmva: utilities to specifying file struct in yaml
tmva: switch data sample

Use a skimmed sample where # of events -> 1/120.  With this factor, we
get ~12k events after preselection for training.  We have ~14k signal
events after preselection from MC.
tmva: add ghost track probability to training vars

Stop gap measure, the variable should be actually max. of all 3 Ds child
ghost track probability.
tmva: presel bugfix, Ds FD χ² & bach mass hypothesis

- both MC & data ntuples have both mass hypotheses: DsK and Dsπ.  Choose
  Dsπ for training.
- Ds flight distance χ² is required to be >=2.  Offline selection
  requires it to be 2 or 9 (KKK or modes with π, Kππ, πππ).  Don't
  change default in skim.cc until confirmed for sure.
tmva: bugfix yaml → file path parser

A separate entry for all directories.  Earlier had an entry for each
leaf directory, the intermediate directories were missing.

  $ python -i yamltest.py
  ['dsk_train_output.root',
   'dsk_train_output.root/InputVariables_PCA',
   'dsk_train_output.root/InputVariables_PCA/CorrelationPlots',
   'dsk_train_output.root/InputVariables_PCA/CorrelationPlots/foo',
   'dsk_train_output.root/InputVariables_Gauss_Deco',
   'dsk_train_output.root/InputVariables_Gauss_Deco/CorrelationPlots',
   'dsk_train_output.root/InputVariables_Id',
   'dsk_train_output.root/InputVariables_Id/CorrelationPlots',
   'dsk_train_output.root/InputVariables_Deco',
   'dsk_train_output.root/InputVariables_Deco/CorrelationPlots']
tmva: add providing titles in yaml
tmva: update sample yaml
tmva: support generic meta-data field in yaml file description
tmva: simpler sample yaml file description
browse.py: bugfix