Reconsider requirement warnings when simulating.

When simulating a CIF file, a warning is printed for every automaton and invariant with a requirement kind. For each invariant the following text is printed:

 WARNING: Invariant <inv> of group <group> is a requirement, but will be simulated as a plant.

It is not uncommon for a file to have 100+ requirement invariants, and the whole console output will be filled with warnings.

This has two downsides:

1. Important warnings may be overlooked.
2. It looks unprofessional when demonstrating the simulation.

Why don't we generate only one warning?

 WARNING: The specification contains requirements, these will be simulated as plants.