* Update roxygen2 doc. Co-authored-by: fis <jm.yuan@outlook.com>
From clang-tidy-9 and gcc-7: Invalid case style, narrowing definition, wrong initialization order, unused variables.
* Apply Configurable to objective functions. * Apply Model to Learner and Regtree, gbm. * Add Load/SaveConfig to objs. * Refactor obj tests to use smart pointer. * Dummy methods for Save/Load Model.