Color sites by family by default in 'kwant.plot'
This will provide more information in the default plot.
This could be implemented in 2 ways:
Assign site families colors in the order that they come out of the system (i.e. the first time a new site family is seen it is assigned the next color in some sequence)
First sort the site families, then assign them colors
The advantage of first sorting is that identical systems constructed in 2 different ways (imagine adding sites from family A before those from B, then another system doing the reverse)
will have the same site coloring, however we would have to iterate over the sites first (
O(N)) in order to get all the site families.