clean up references and smart-ptrs

[ichthyo]

This is a tracking Ticket for achieving a more uniform handling of (language) references and smart-ptrs within the Proc-Layer [BR]

Guidelines

• for freely-accessible operations, smart-ptrs should be preferred
• functions to be executed in a controlled scope (esp. regarding parallelism)
  • express the fact of taking ownership by using a smart-ptr
  • otherwise preferably should take a (language) const ref
• we should try to find a uniform naming scheme, esp. for typedefs denoting smart ptrs

[ichthyo]

taking responsibility for overviewing this task, while deferring it for later.