Use integer division when calculating DynamicStringIndicesDataField count.

Use integer division when calculating DynamicStringIndicesDataField count.

In Python 3, the floating point division lead to:

    Traceback (most recent call last):
      ...
      File ".../igor/struct.py", line 255, in unpack_data
        items = [next(iterator) for i in range(self.arg_count)]
    TypeError: 'numpy.float64' object cannot be interpreted as an integer