StartVector initializes bookkeeping for writing a new vector.

        A vector has the following format:
          - <UOffsetT: number of elements in this vector>
          - <T: data>+, where T is the type of elements of this vector.

def StartVector(self, elemSize, numElems, alignment):
        """
        StartVector initializes bookkeeping for writing a new vector.

        A vector has the following format:
          - <UOffsetT: number of elements in this vector>
          - <T: data>+, where T is the type of elements of this vector.
        """

        self.assertNotNested()
        self.nested = True
        self.Prep(N.Uint32Flags.bytewidth, elemSize*numElems)
        self.Prep(alignment, elemSize*numElems)  # In case alignment > int.
        return self.Offset()