Computes the inverse of a matrix that has been decomposed into u,' w' and
v' by singular value decomposition. It assumes the small'' elements of w'
have already been zeroed. It computes the inverse by constructing a diagonal
matrix w2' from w' (which is just a vector of the diagonal elements, and then
explicitly multiplying u^t w2 and v. Note that if you are computing the inverse
merely to solve one or more systems of equations, you are better off using the
decomposition and backsubstitution routines directly.