freeze_orbitals reflects the prune flag (#803)

* Update operator_tapering.py

Make the call to prune_unused_indices in freeze_orbitals conditional on the 'prune' kwarg

* Update operator_tapering.py

Updated description for freeze_orbitals

* Update operator_tapering.py

remove trailing whitespace

---------

Co-authored-by: Mafalda Ramôa Alves <74077940+MafaldaRA@users.noreply.github.com>
Co-authored-by: Matthew Harrigan <mpharrigan@google.com>

Author: mafaldaramoa

src/openfermion/transforms/repconversions/operator_tapering.py

@@ -21,16 +21,18 @@
 def freeze_orbitals(fermion_operator, occupied, unoccupied=None, prune=True):
     """Fix some orbitals to be occupied and others unoccupied.
 
-    Removes all operators acting on the specified orbitals, and renumbers the
-    remaining orbitals to eliminate unused indices. The sign of each term
-    is modified according to the ladder uperator anti-commutation relations in
-    order to preserve the expectation value of the operator.
+    Removes all operators acting on the specified orbitals, and optionally
+    renumbers the remaining orbitals to eliminate unused indices. The sign of
+    each term is modified according to the ladder operator anti-commutation
+    relations in order to preserve the expectation value of the operator.
 
     Args:
         occupied: A list containing the indices of the orbitals that are to be
             assumed to be occupied.
         unoccupied: A list containing the indices of the orbitals that are to
             be assumed to be unoccupied.
+        prune: A flag that determines whether the orbitals will be renumbered to
+            eliminate unused indices.
     """
     new_operator = fermion_operator
     frozen = [(index, 1) for index in occupied]
@@ -81,8 +83,9 @@ def freeze_orbitals(fermion_operator, occupied, unoccupied=None, prune=True):
                 if op[0] > index:
                     new_operator.terms[term] *= -1
 
-    # Renumber indices to remove frozen orbitals
-    new_operator = prune_unused_indices(new_operator)
+    if prune:
+        # Renumber indices to remove frozen orbitals
+        new_operator = prune_unused_indices(new_operator)
 
     return new_operator