Returns V, a weigthed sample of K elements taken among X without replacement
X a vector of numerics
K amount of element to sample from x
W a vector of positive weights w, whose length is length(x)
% EXAMPLE:
% for i=1:100
% v(i)=randsampleWRW([0,0.5,3,20],1,[0.5,0.4,0.05,0.05]);
% end
% plot(v,'o')
