[mlfinlab] Change of name and naming conventions for trend_scanning_labels(), get_events() and get_b

I believe the current naming conventions used for ```trend_scanning_labels()``` are misleading. This function in effect returns: - events (the 't1' column) just like the ```get_events()``` function - associated returns (the 'ret' column) and bins just like the ```get_bin()``` function So the resulting dataframe can be used for return or time decay weights generation, for primary labelling, as well as for meta-labelling. This flexibility is really great, but I fear some users might not realise it. The "labels" part of the name is actually a placeholder for both "events" and "bins". I'm not sure how to best tackle this, but I would suggest first changing the name to a ```get_trend_scanning_labels()``` format to be in line with the ```get_events()``` and ```get_bins()``` naming convention. As for the rest of the name, ```get_trend_scanning_events_and_bins()``` would be more precise but too lengthy. So I would suggest: - either splitting the function in two like for tr