Documentation of KZGSettings (Branch das)

[b-wagn]

Note: this issue is about this branch.

When reading the code, I felt that the different fields related to roots of unity in KZGSettings (here) are a bit confusing.
This includes the fields roots_of_unity, expanded_roots_of_unity, and reverse_roots_of_unity.

As a result, it is a bit hard to understand code that uses these fields, or to modify code properly if needed.
For example, here are some questions that I can not immediately resolve from the documentation.
A better documentation and more consistent naming could help to identify what they are.
The main questions that the comments/naming should address for each field are the following:

• Is the field related to a blob or to an extended blob?
• How long is the field, i.e., how many elements does it contain?
• Are these roots in reverse bit ordering or not?