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 taking advantage of the
known zeros in the full 2-dimensional `w' matrix. It uses the backsubstitution
algorithm, only with the B vectors fixed at the columns of the identity matrix,
which lets us take advantage of its zeros. It's about twice as fast as the slow
version and conses a lot less. 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.