Interface refactoring

Find a design which is more intuitive and minimal.

examples/testInterface.ipynb

+%% Cell type:code id: tags:
 
+``` python
+import numpy as np
+from codes import utils, model, solvers
 ```
 
+%% Cell type:code id: tags:
 
+``` python
+def random_guess(vectors, int_model):
+    """
+    Generate random guess.
+
+    Parameters:
+    -----------
+    vectors : list of tuples
+        Hopping vectors for the mean-field corrections.
+    """
+    scale = 0.1*(1+np.max(np.abs([*int_model.values()])))
+    return utils.generate_guess(
+        vectors=vectors,
+        ndof=len([*int_model.values()][0]),
+        scale=scale
+    )
 ```
 
+%% Cell type:code id: tags:
 
+``` python
+U0 = 1
+nk = 100
+filling = 2
+
+hopp = np.kron(np.array([[0, 1], [0, 0]]), np.eye(2))
+tb_model = {(0,): hopp + hopp.T.conj(), (1,): hopp, (-1,): hopp.T.conj()}
+int_model = {
+    (0,): U0 * np.kron(np.eye(2), np.ones((2, 2))),
+}
+
+testModel = model.MfModel(tb_model, int_model, filling)
+guess = utils.tb2grid(random_guess(tb_model.keys(), int_model), nk)
+solvers.kspace_solver(testModel, guess)
 ```
 
+%% Output
 
+    array([ 4.81536544e-01,  6.41071801e-03, -1.76048506e-07, ...,
+            0.00000000e+00,  1.33679796e-06,  0.00000000e+00])
 
+%% Cell type:code id: tags:
 
+``` python
 ```
 
+%% Cell type:code id: tags:
 
+``` python
 ```