PLCgen: Enable generating PLC code comments from user-added descriptions

When developing a CIF specification for a real-world system, various parts of the specification quite directly relate to real-world concepts such as building blocks in the system or requirements / needs that are covered from functional, legal, or other perspectives.
When generating PLC code from such a specification, it would be useful to be able to annotate such parts in the generated code to make the code more easy to understand and/or review.

As conceptual very sunny-weather idea (that is, ignoring all implementation restrictions):

From

// R1: Ensure at least one pump is working.
requirement pump1.on or pump2.on;

to

case event_pump1_off:
   (* R1: Ensure at least one pump is working. *)
   IF PUMP2_LP == ON THEN
      PUMP1_LP := OFF;
   END_IF;

case event_pump2_off:
   (* R1: Ensure at least one pump is working. *)
   IF PUMP1_LP == ON THEN
      PUMP2_LP := OFF;
   END_IF;

What needs to be discussed first is what can be done here.

Addresses #397 (closed)