implement more efficient wsample
add type restrictions
switch between algorithms, shuffle by default
fix wsample usage
added MGF and characteristic function for NoncentralChisq. Reference
Johnson, Kotz, Balakrishnan, C.U.D. Vol. I pg. 437
added MGF and characteristic function for Logistic. Reference
Johnson, Kotz, Balakrishnan, C.U.D. Vol. I pgs. 116, 117