title: weight calculation of individual tree in XGBoost when using "binary:logistic"
body: <p>Taking cue from <a href="https://stackoverflow.com/questions/32950607/how-to-access-weighting-of-indiviual-decision-trees-in-xgboost">How to access weighting of indiviual decision trees in xgboost?</a>.
  How do one calculate the weights when objective = "binary:logistic", and eta = 0.1? </p>
  
  <p>My tree dump is:</p>
  
  <pre><code>booster[0]
  0:[WEIGHT&lt;3267.5] yes=1,no=2,missing=1,gain=133.327,cover=58.75
      1:[CYLINDERS&lt;5.5] yes=3,no=4,missing=3,gain=9.61229,cover=33.25
          3:leaf=0.872727,cover=26.5
          4:leaf=0.0967742,cover=6.75
      2:[WEIGHT&lt;3431] yes=5,no=6,missing=5,gain=4.82912,cover=25.5
          5:leaf=-0.0526316,cover=3.75
          6:leaf=-0.846154,cover=21.75
  booster[1]
  0:[DISPLACEMENT&lt;231.5] yes=1,no=2,missing=1,gain=60.9437,cover=52.0159
      1:[WEIGHT&lt;2974.5] yes=3,no=4,missing=3,gain=6.59775,cover=31.3195
          3:leaf=0.582471,cover=25.5236
          4:leaf=-0,cover=5.79593
      2:[MODELYEAR&lt;78.5] yes=5,no=6,missing=5,gain=1.96045,cover=20.6964
          5:leaf=-0.643141,cover=19.3965
          6:leaf=-0,cover=1.2999
  </code></pre>