Interface refactoring

Find a design which is more intuitive and minimal.

codes/kwant_helper/kwant_examples.py

 import kwant
 import numpy as np
+
 from codes.kwant_helper.utils import build_interacting_syst
 
 s0 = np.identity(2)
 sz = np.diag([1, -1])
 
+
 def graphene_extended_hubbard():
+    """Create a graphene system with extended Hubbard interactions.
+
+    Return:
+    ------
+    bulk_graphene : kwant.builder.Builder
+        The bulk graphene system.
+    syst_V : kwant.builder.Builder
+    """
     # Create graphene lattice
     graphene = kwant.lattice.general(
-        [[1, 0], [1 / 2, np.sqrt(3) / 2]], [[0, 0], [0, 1 / np.sqrt(3)]],
-        norbs=2
+        [[1, 0], [1 / 2, np.sqrt(3) / 2]], [[0, 0], [0, 1 / np.sqrt(3)]], norbs=2
     )
     a, b = graphene.sublattices
 
@@ -23,14 +32,14 @@ def graphene_extended_hubbard():
 
     def onsite_int(site, U):
         return U * np.ones((2, 2))
+
     def nn_int(site1, site2, V):
         return V * np.ones((2, 2))
 
     syst_V = build_interacting_syst(
         builder=bulk_graphene,
-        lattice = graphene,
-        func_onsite = onsite_int,
-        func_hop = nn_int,
+        func_onsite=onsite_int,
+        func_hop=nn_int,
     )
-    
-    return bulk_graphene, syst_V
\ No newline at end of file
+
+    return bulk_graphene, syst_V