Read the first 'size' bytes in packet and advance cursor past them.

def read(self, size):
        """Read the first 'size' bytes in packet and advance cursor past them."""
        result = self._data[self._position:(self._position+size)]
        if len(result) != size:
            error = ('Result length not requested length:\n'
                     'Expected=%s.  Actual=%s.  Position: %s.  Data Length: %s'
                     % (size, len(result), self._position, len(self._data)))
            if DEBUG:
                print(error)
                self.dump()
            raise AssertionError(error)
        self._position += size
        return result