Tried to fix lanczos implementation

Still solution far from correct.

tests/test_main.py

@@ -0,0 +1,27 @@
+import numpy as np
+import numpy.linalg as LA
+from afgl.lanczos import lanczos
+
+"""
+Todo: better test case
+"""
+
+
+def test_lanczos_return_correct_solution():
+    N = 6
+    M = 4
+
+    A = np.random.randint(1, 10, size=(N, N))
+    L = (A + A.T) / 2
+    s = np.random.randint(1, 10, N)
+    [V, alp, beta] = lanczos(L, s, M)
+
+    T = np.diag(alp) + np.diag(beta, -1) + np.diag(beta, 1)
+
+    x = LA.solve(A, s)
+    e_1 = np.zeros(M)
+    e_1[0] = 1
+    y = (LA.inv(T) @ e_1) * LA.norm(s)
+    x_lanczos = V @ y
+
+    assert LA.norm(x - x_lanczos) < 1e-3