'Returns the i\'th point of a series
If start point is negative infinity, point is returned from the end.
Assumes the first point to be indexed zero.
Examples
TODO'

def _ith_point(self, i):

    if (self.start is S.NegativeInfinity):
        initial = self.stop
        step = (-1)
    else:
        initial = self.start
        step = 1
    return (initial + (i * step))
