Summary

Today in the R package, there are a lot of internal function calls which use only positional arguments. Change them to use keyword arguments for extra safety.

I've added this issue to provide a small, focused contribution opportunity for Hacktoberfest 2020 participants. If you are an experienced open source contributor, please leave this

Problem:
catboost version: 0.23.2
Operating System: all
Tutorial: https://github.com/catboost/tutorials/blob/master/custom_loss/custom_metric_tutorial.md

Impossible to use custom metric (С++).

Code example

from catboost import CatBoost
train_data = [[1, 4, 5, 6],