Release 0.35.0

Changelog

• added generic model navigation functions
• added special handling for plantuml c4 deployment diagrams
• extended external? criterium to relations
  • a relation is considered external, if any of the related nodes is external
• refactored model node collection for views
• fixed parent index for modelled :contained-in relations
• fixed missing sprites in plantuml c4 diagrams
• fixed model test data
• enhanced doc templates
• added CONTRIBUTING document