FastCSMatrix: The operator form of the contraint matrix for the fast compressed sensing problem.

Specifically, it returns y = A*x (mode = 1) or y = A *x (mode = 2), where A is an mxn matrix given by A = [Phi - Phi], and Phi = P*H*Q, where P,Q are random permutation matrices, and H is a fast Hadamard/Fourier operator.