Add set_internal_tuning() helper method for configuring internal hyperparameter optimization

[sebffischer]

Configuring the internal tuning of a Learner that supports it is somewhat tricky because one has to read through the lengthy parameter documentation. We (marc, bernd, me) decided to add a S3 generic set_internal_tuning(learner, ...) which makes this easier.

For xgboost, e.g. this would look something like:

set_internal_tuning.LearnerXgboost = function(learner, validate, early_stopping_rounds, nrounds, eval_metric) {
  # 1. set those parameters that were specified by the user in the learner
  # 2. afterwards check that early stopping is now properly enabled.
}