Broken GaussianEnergy

Sampling with a Gaussian energy and no mean was broken in one of the recent merge request.

Applying the following path

diff --git a/demos/getting_started_3.py b/demos/getting_started_3.py
index 2dcffa17..c053e0a0 100644
--- a/demos/getting_started_3.py
+++ b/demos/getting_started_3.py
@@ -109,8 +109,7 @@ if __name__ == '__main__':
     minimizer = ift.NewtonCG(ic_newton)

     # Set up likelihood and information Hamiltonian
-    likelihood = (ift.GaussianEnergy(mean=data, inverse_covariance=N.inverse) @
-                  signal_response)
+    likelihood = ift.GaussianEnergy(inverse_covariance=N.inverse) @ ift.Adder(data, neg=True) @ signal_response
     H = ift.StandardHamiltonian(likelihood, ic_sampling)

     initial_mean = ift.MultiField.full(H.domain, 0.)

results in the demo failing though it actually should do the exact same thing as before.