prevent invalid sub requirements

[jpalnick]

add code to ensure that it is not possible to:

• add a open child to a closed parent
• reopen a child of a closed parent
• setting a child to the backlog
• setting a child to an iteration that is after the iteration of its parent or higher
• setting a parent to an iteration before any of its children
• add a child to a parent that is its descendant (cyclic dependency)
• add a child having a parent, as a child of another parent (forest not preserved)

this is an extension of issue #81

[tuna-salad]

Cyclic dependency causes a big trouble. It seems to be stuck in an infinite loop, using more and more memory, so that I had to close the program before its RAM use reached 2GB. The cycle was also saved into my database, so I had to create it again.