Add explanatory text to nested-logit notebook

[EwoutH]

Add some explanatory notes to the 01-logit-nested.ipynb notebook. The goal hereby is to explain the concepts behind the code. Also adds a introduction, some interpretation of the results, and a summary.

I hope this is useful, please review on both correctness and readability. Please also let me know if you like this level of detail (or you would like to see more or less).