nim sum

function that computes the nim sum

% nimsum(a,b) returns the nim sum of a and b
% nimsum(V) returns the nim sum of the elements of V
% nimsum(M) returns a vector with the nim sums of the columns of M
% nimsum(V,a) returns a vector with the nim sums of V(i) and a
% nimsum(a,V) returns a vector with the nim sums of a and V(i)
% nimsum(M,a) returns a matrix with the nim sums of M(i,j) and a
% nimsum(a,M) returns a matrix with the nim sums of a and M(i,j)
% nimsum(V,W) returns a vector with the nim sums of V(i) and W(i)
% nimsum(M,N) returns a matrix with the nim sums of M(i,j) and N(i,j)