digital sinc function or Direchlet kernel
[s]=sincd(n,flag)
:n integer : :flag if flag = 1 the function is centred around the origin; if
flag = 2 the function is delayed by %pi/(2*n)
: :s vector of values of the function on a dense grid of frequencies :
function which calculates the function Sin(N*x)/N*Sin(x)
`plot`_(sincd(10,1))