Rename (open, closed) targets to avoid confusion

[MattWindsor91]

I'm proposing changes to the RoboCert metamodel that will overload the meaning of 'open' and 'closed' in terms of targets to mean targets that represent opaque or transparent projections of RoboChart entities. As such, the current use (whether or not the parametrisation is left open for amendment) will become even more confusing than it is.

I propose renaming as follows:

• Open targets (ones with parameters left open) become Definition, eg the raw definition of the target;
• Closed targets become Instantiated targets.

This should be a quick renaming - possibly just of two constants.

[MattWindsor91]

We're no longer making those change proposals, but I still think this rename might be helpful.