Improve user experience when dealing with densities
The original Kwant made it quite easy to define TB systems in terms of sites and to evaluate transport properties like transmission probabilities between leads. In this workflow low-level systems and their inner workings were an implementation detail of Kwant.
When the operator module was added, it was natural for it to deal with low-level systems. However this meant that suddenly the user was exposed to details of low-level systems like the order of sites and their indices.
We would like to provide an intuitive way to handle densities, and the vectorization effort seems like a good moment to introduce a new improved API because we have to review operators for ND systems anyway.
To proposals exist that offer different solutions to this problem:
I propose to discuss the topic generally here and decide on a course of action. At the end of the discussion, one of the above two issues could be updated, and the other closed (along with this one).