Implement Bootstrapping

[mbillingr]

Add an option to train on bootstrap resamples of the training data.

Pros:

• Similar to the original Random Forest
• Can speed up fitting by using less samples

Cons:

• Requires either more copying of data or another indirection layer to acess data

Open questions:

Resolved questions:

• Who should do the reampling - the forest builder before passing the data to the tree builder, or the tree builder after it got passed the data? I don't think it matters for the current sequential implementation, but it may be relevant for parallelization in the future. The tree builder should do the resampling.