support different column names for ids, times and targets

[jmoralez]

• Adds support for providing id_col, time_col and target_col to the methods that take a dataframe so that different column names can be used, i.e. removes the restriction on unique_id, ds, y.
• Makes the forwarding of the arguments to the distributed backends explicit.
• Removes the distributed.utils module, which doesn't seem to be used anymore along with the _nbdev.py file (from nbdev1).
• Fixes some docstrings but I'll make another PR to define them only once and set them in all of the methods to make it easier and ensure they're consistent. Templates the common arguments in the docstrings to ensure consistency and to make it easier to update them.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[AzulGarza]

Thank you, @jmoralez! This is a great improvement. I just left a comment regarding the documentation of X_df.

[AzulGarza]

🎉