FIX: import catboost module if present, else create local module mockup

[j-ittner]

This PR makes sure that catboost is properly defined in the local namespace of private module _explainer.py. No impact on the public API, but eliminates a warning during the Sphinx documentation build.

[bhadurishirsesh]

Understood the change, want to understand the need for try except in the import step

[j-ittner]

Understood the change, want to understand the need for try except in the import step

catboost is not a required module, so we use try: … except: … to gracefully handle the case that catboost is not available