UVL: Lacking support for multiplicities on features

[joeriexelmans]

Prerequisitives

• Eclipse Version: Eclipse Modeling Tools 2021-06 (4.20.0)
• FeatureIDE-Version: 3.8.0
• Operating system: NixOS 21.05 (x64, Linux kernel 5.10.66, GNOME 40.1.0, X11)
• Java runtime used to run eclipse: OpenJDK 11
• FeatureIDE Composer: Add text here (optional, leave blank if not needed)
• Git branch/Commit-ID: Add text here (optional, leave blank if not needed)
• Java compiler used to build the FeatureIDE plugin: Add text here (optional, leave blank if not needed)

Issue description

According to Engelhardt's master thesis, in UVL, features can be given multiplicities, such that a feature can be present more than once.

When adding a multiplicity to the UVL source, in the graphical view it is treated as an optional feature, and when making a change to the graphical view, it is overwritten as an optional feature.

[rahelsundermann]

Multiplicities for features are generally not supported by FeatureIDE, even though they are possible with UVL. Anyway, FeatureIDE should probably forbid (with an appropriate error message) to switch to the graphical editor in such a case.