Interface refactoring

Find a design which is more intuitive and minimal.

codes/kwant_helper/kwant_examples.py

 import kwant
 import numpy as np
-from .kwant_helper.utils import build_interacting_syst
+from codes.kwant_helper.utils import build_interacting_syst
 
 s0 = np.identity(2)
 sz = np.diag([1, -1])