PINS state and label manipulation functionality

[Meijuh]

Add highlevel functions/types to PINS for:

StateVariable : Name, Type, GroupDependencies

addStateVariable (model, int index, variable)

addStateLabel (model, int index, ltsmin_expression)

These function can greatly reduce the amount of code in the front-end glue code (e.g. prom-pins.c) and pins2pins wrappers.

Note that an ltsmin_expression can function as a (boolean) state label. Once added, labels can be reused in other predicates (See mark_predicate function for how the labels are treated as state variables).

SpinS already exports guards and other labels (progress, accept and valid end) in the correct way.

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Alfons: My latest POR branch contains a pins-util.ch files with some initial PINS helper functions