examples for loops.

The syntax is listed in the ngspice manual,
chapter 17.6 Control Structures. Practical examples
using a simple voltage divider circuit are given here.

examples/loops/loop_while.cir

@@ -0,0 +1,12 @@
+example while loop
+.control
+
+let loopindex = 0
+while loopindex < 5
+  echo index is $&loopindex
+  let loopindex = loopindex + 1
+end
+
+.endc
+
+.end