v1.6.0

Wrappers for lifelines library models are implemented in the "external" module.
Built new structure of leaf models. Now, we can use external models as leaf_model in tree-based approaches. Parameters for leaf_model set as "leaf_" at tree initialization step.
Increased performance of CRAID and BootstrapCRAID models.
Added new Parallel Bootstrap CRAID model (works with n_jobs parameter).