rollu - Roll the three top stack elements upward
obj1 obj2 obj3 rollu -> obj3 obj1 obj2
rollu is equivalent to 3 1 roll
Raises StackUnderflow error if there are less
than 3 objects on the stack.
/builddir/build/BUILD/nest-simulator-3.3/nest-simulator-3.3/sli/slistack.cc