[tet.py] Allow user to set hamiltonian term on the same positions mul…

…tiple times.

It allows user to use `state.hamiltonians[positions]+=tensor`.
Previously, the program will throw error saying hamiltonian term has
already been set, but recently, some models need adding term on the
same positions.

CHANGELOG.org

@@ -13,6 +13,9 @@
   without read all configurations.
 + *tetragono*: Yield measurement result before save state and configuration in gradient descent of sampling lattice.
   Since user may want to update attributes out of yield.
++ *tetragono*: Allow user to set hamiltonian term on the same positions multiple times, which allows user to use
+  =state.hamiltonians[positions]+=tensor=. Previously, the program will throw error saying hamiltonian term has already
+  been set, but recently, some models need adding term on the same positions.
 *** Deprecated
 *** Removed
 + *tetragono*: Remove ansatz product state support, which may be added again in the future, but it require many updates.

tetragono/tetragono/abstract_state.py

@@ -455,8 +455,6 @@ def _set_hamiltonian(self, points, tensor):
                 raise ValueError("Wrong hamiltonian edge")
             if edge_out.conjugated() != edge_in:
                 raise ValueError("Wrong hamiltonian edge")
-        if points in self._hamiltonians:
-            raise RuntimeError("This hamiltonian term is already set")
 
         if tensor.norm_max() != 0:
             self._hamiltonians[points] = tensor