WIP: Relax domain checking to use only `==`

Instead of comparing the instances of the main, merely check for equality, i.e. make sure both objects describe the same domains.

nifty5/minimization/metric_gaussian_kl.py

@@ -83,7 +83,7 @@ class MetricGaussianKL(Energy):
 
         if not isinstance(hamiltonian, StandardHamiltonian):
             raise TypeError
-        if hamiltonian.domain is not mean.domain:
+        if hamiltonian.domain != mean.domain:
             raise ValueError
         if not isinstance(n_samples, int):
             raise TypeError