Fix sympy coefficient typing issue in SymbolicOperator

When adding a new term to a SymbolicOperator with a sympy coefficient, the coefficient was being cast to a float. This was because the default value for a new term was 0.0, which is a float. This commit changes the default value to 0, which is an integer. This ensures that the type of the coefficient is preserved when adding new terms.

Author: mhucka

src/openfermion/ops/operators/symbolic_operator.py

@@ -433,7 +433,7 @@ def __iadd__(self, addend):
         """
         if isinstance(addend, type(self)):
             for term in addend.terms:
-                self.terms[term] = self.terms.get(term, 0.0) + addend.terms[term]
+                self.terms[term] = self.terms.get(term, 0) + addend.terms[term]
                 if self._issmall(self.terms[term]):
                     del self.terms[term]
         elif isinstance(addend, COEFFICIENT_TYPES):