Builder fixes

Address issues related to interfacing with kwant, such as qt/kwant-scf#10 and qt/kwant-scf#9.

meanfi/tests/test_kwant.py

@@ -42,7 +42,7 @@ def test_kwant_conversion(seed):
         random_tb, list(dummy_tb.sites()), dummy_tb.symmetry.periods
     )
     # convert builder back to tb and compare
-    random_builder_tb = builder_to_tb(random_builder, return_data=True)
+    random_builder_tb = builder_to_tb(random_builder)
     compare_dicts(random_tb, random_builder_tb)
 
 
@@ -51,7 +51,6 @@ def test_kwant_supercell(seed):
     """Test with Kwant supercell and callable onsite and hoppings."""
     np.random.seed(seed)
     ndim = np.random.randint(1, 3)
-    cutoff = np.random.randint(1, 3)
     sites_in_cell = np.random.randint(1, 4)
     ndof_per_site = [np.random.randint(1, 4) for site in range(sites_in_cell)]
     n_cells = np.random.randint(1, 4)