add new problem to textbook

[rsonecha]

This change is 

[liruiw]

Nit. In this problem you will further explore Dense Object Nets, which were introduced in lecture. Dense Object Nets are able to quickly learn consistent pixel-level representations for visual understanding and manipulation. Dense Object Nets are powerful because the representations they predict are applicable to both rigid and non-rigid objects. They can also generalize to new objects in the same class and can be trained with self-supervised learning.