non-"NoVal"able ports
Allow the user to indicate that some ports cannot be equal to NoVal.
Entails:
- GUI: check box in the properties to say “No NoVal”
- Adaptation of the simulation: last value should be kept
- Adaptation of code generation: last value should be kept
- Adaptation of transformation to NuSMV/nuXmv
(from redmine: issue id 2561, created on 2016-04-19, closed on 2018-03-05)