Adding a relay/schmitt tigger operation

Possibly a controversial contribution; an operation with side-effects (state).
The state is managed by the work vector of the XFunctionInternal.

Idea:
A relay (schmitt-trigger) operation, for modeling hysteresis

A relay will introduce memory into an algorithm.
The output of the relay (1 or -1) depends on the history (state) and the current input.

 If state==true:  output = state = (input>-1)
 If state==false:  output = state = (input<1)

````
                ^  output
            ____|___________  1 (true)
           |    |     |
   --------+----|-----+------------>  input
    -1 ____|____|_____|
   (false) -1         1
  ```
  The initial state is always true (high)