Hierarchical platform: metamodel extensions

This issue groups minor (backwards compatible) extensions of the hierarchical platform metamodel

• Enable direct connections between OffChipNetworks
• Introduce Sensor and Actuator elements that can be connected to offchip networks
• Enable modelling of the accessibility (e.g., private, public) of platform elements
• Remove outdated platform architecture annotations
  • Add model migrators
  • While here, remove model migrators that have been active for at least 2 releases (keep generic ones)
  • Apply model migrators to all test & example models