Add numbers.Number to COEFFICIENT_TYPES in SymbolicOperator (#1171)

numpy.int64 and other numeric types are currently not accepted as
coefficients (assuming this is not on purpose). Instead of adding them
manually, numbers. Number evaluates to true for all of them.

In principle int, float, complex could be removed (left them for
clarity)

This prevents errors like
```python
    def __init__(self, term=None, coefficient=1.0):
        if not isinstance(coefficient, COEFFICIENT_TYPES):
>           raise ValueError('Coefficient must be a numeric type. Got {}'.format(type(coefficient)))
E           ValueError: Coefficient must be a numeric type. Got <class 'numpy.int64'>
```

Author: kottmanj

src/openfermion/ops/operators/symbolic_operator.py

@@ -16,12 +16,13 @@
 import itertools
 import re
 import warnings
+import numbers
 
 import sympy
 
 from openfermion.config import EQ_TOLERANCE
 
-COEFFICIENT_TYPES = (int, float, complex, sympy.Expr)
+COEFFICIENT_TYPES = (int, float, complex, sympy.Expr, numbers.Number)
 
 
 class SymbolicOperator(metaclass=abc.ABCMeta):