twoProduct
Returns the exact result of multiplying two doubles.
the resulting array is the reverse of the standard twoSum in the literature.
Theorem 18 (Shewchuk): Let a and b be p-bit floating-point numbers, where p >= 6. Then the following algorithm will produce a nonoverlapping expansion x + y such that ab = x + y, where x is an approximation to ab and y represents the roundoff error in the calculation of x. Furthermore, if round-to-even tiebreaking is used, x and y are non-adjacent.
See https://people.eecs.berkeley.edu/~jrs/papers/robustr.pdf
Parameters
a
A double
b
Another double