title: is there a way to set the "base margin" for XgBoost in the MLR framework?
body: <p>I am trying to fit an XgBoost model within the MLR framework. While the framework is fairly well documented, there are some specifics of the XgBoost library that I cannot replicate within MLR, one in particular being the "base margin". 
  In XgBoost library I would just set :</p>
  
  <pre><code>db_xgbmatrix &lt;- xgb.DMatrix(db)
  setinfo(db_xgbmatrix , "base_margin", margin)
  </code></pre>
  
  <p>and then I can just go on with the training of the model; whereas, in MLR, once I create the task and the learner: </p>
  
  <pre><code>tsk      &lt;- makeRegrTask(data =  db, target = target_var)
  lrn &lt;- makeLearner("regr.xgboost", predict.type = "response", eta = 0.1,
                     max_depth = 8, min_child_weight = 20,
                     subsample = 0.75, colsample_bytree = 0.75,
                     nrounds = 100, nthread = cl_n, objective="count:poisson")
  </code></pre>
  
  <p>I'm not quite sure of where the base margin should be set. Any ideas? Is that feature implemented, and hidden somewhere?
  Thank you all in advance</p>