HADAMARD(N) is a Hadamard matrix of order N, that is,
a matrix H with elements 1 or -1 such that H'*H = N*EYE (N).
An N-by-N Hadamard matrix with N > 2 exists only if REM (N,4)=0.
This function handles only the cases where N, N/12 or N/20
is a power of 2.