Fix GitHub Actions, R CMD Check and some tests

[mikolajsp]

This PR is meant to fix the automatic checks that verify correctness of the package implementation.

Changes:

• Fixes to GitHub actions - using the newest versions of r-lib/actions.
• Changes to DESCRIPTION - catboost listed as part of "Suggests", with "Remotes" and "Additional_repositories" fields added, which allow it to be installed for the GitHub actions check.
• Change class checking in unify functions - use inherits() instead of class() == "class" to make R CMD Check pass.
• Make the tests use the proper temp directory for saving models to make R CMD Check pass.
• Add skip_if_no_catboost() function in the tests to cover the situation when catboost is not installed.

[pbiecek]

thank you!