Add iddata implementation for prbs signals

src/idinput.jl

+function idinput(N, class = "prbs"; band = 1)
+    output = zeros(N)
+    if lowercase(class) == "prbs"
+        start = 0x02;
+        a = start;
+        for i = 1:N
+            newbit = (((a >> 12) $ (a >> 3) $ (a >> 2) $ (a >> 0)) & 1);
+            a = ((a << 1) | newbit) & 0x7f;
+            output[i] = newbit
+        end
+        if band > 1
+            output = repmat(output',band,1)[1:N]
+        end
+    else
+        warn("Input class $class not supported")
+    end
+
+
+    return output
+end
+