Fix complex samples

One of the possible ways to solve #306 (closed) . It makes the Hamiltonian consistent with the sampels from the Gaussian distribution, but now for complex Gaussian samples

H(s) = -\frac{1}{2}s^\dagger S^{-1} s
<ss^\dagger>=S

is nor satisfied anymore. Instead we have

<ss^\dagger> = 2S

test/test_operators/test_fisher_metric.py

@@ -30,6 +30,7 @@ field = list2fixture([ift.from_random(sp, 'normal') for sp in spaces] +
         [ift.from_random(sp, 'normal', dtype=np.complex128) for sp in spaces])
 
 Nsamp = 2000
+np.random.seed(42)
 
 def _to_array(d):
     if isinstance(d, np.ndarray):