Fix 2 problems in _multitensor.py

The `dual_basis` parameter to the `MultiTensor` class constructor was
defaulted to `DualBasis()`. In Python, default arguments are evaluated
only once at definition time, meaning every `MultiTensor` instance
created without an explicit basis shared the same global `DualBasis`
object. When tests were run in parallel or repeated, data from one test
would leak into others, leading to errors.

In addition, `MultiTensor.add_dual_elements` used `list.extend()`
instead of `list.append()` when adding `DualBasisElement` objects. Since
`DualBasisElement` is iterable (yielding its internal terms), using
`extend()` incorrectly decomposed the object into its constituent tuples
instead of adding the object as a single unit. This caused
`MultiTensor.synthesize_dual_basis()` to fail in some cases.

Author: mhucka

src/openfermion/contrib/representability/_multitensor.py

@@ -23,7 +23,7 @@ def __iter__(self):
 
 
 class MultiTensor(object):
-    def __init__(self, tensors, dual_basis=DualBasis()):
+    def __init__(self, tensors, dual_basis=None):
         """
         A collection of tensor objects with maps from name to tensor
 
@@ -46,6 +46,8 @@ def __init__(self, tensors, dual_basis=DualBasis()):
         self.off_set_map = self.make_offset_dict(self.tensors)
 
         # An iterable object that provides access to the dual basis elements
+        if dual_basis is None:
+            dual_basis = DualBasis()
         self.dual_basis = dual_basis
         self.vec_dim = sum([vec.size for vec in self.tensors])
 
@@ -82,7 +84,7 @@ def add_dual_elements(self, dual_element):
             raise TypeError("dual_element variable needs to be a DualBasisElement type")
 
         # we should extend TMap to add
-        self.dual_basis.elements.extend(dual_element)
+        self.dual_basis.elements.append(dual_element)
 
     def synthesize_dual_basis(self):
         """