cleanup: src/lib/coord - PNode System

[patrick]

Clean up the source, as stated in the ​cleanup milestone

• PNode is not reliable, it doesn't do what the user expects it to do (because it is only updated after each mainloop-cycle)
• deleting PNodes: make delete operator private/protected, so pnodes can not be deleted by accident
• create a new delete function e.g. delete() that is used for Pnode internal cleanup ⇒ this will have very critical effects on the whole framework
• check for better PNode tree integrity