Add numbers.Number to COEFFICIENT_TYPES in SymbolicOperator

[kottmanj]

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

    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'>

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