Parallelization

[martinju]

We should add 4 arguments for parallelization:

1. One concerning parallelization of the predictions method (which is passed to prediction_vector)
2. One for parallelization of the sampling method when either the Gaussian or copula method is used.
3. One which concerns parallelization over test samples in compute_kshap
4. One for parallelization of distance computation in prepare_kshap.

We should also add a test checking that either 3 or both of 1 and 2 are set to 1 core to avoid parallelization within parallelizations.

[martinju]

We have implemented parallellization within the prepare_data function of class ctree on the ctree branch, see f8734aa.

[martinju]

Currently, the approach in #244 seems to be the best way to handle this.