First steps towards fix: locking layout while recursive shape iterato… …

…r is alive.

Releasing the lock when the iterator is at end - otherwise a finished… …

… iterator could still block the layout (potential Ruby GC issue)

Debugging, added tests for new recursive shape iterator.

Solving potential thread collision issues upon Layout::update.

Avoiding a layout lock on Region::empty.

Fixed some Ruby tests - as RecursiveShapeIterator lifelime may be lon… …

…ger than expected, we need to explicitly destroy the iterator now to avoid layout locking.

Also provide layout locking for recursive instance iterator

Adding forced update to Layout writer to ensure proper files. A warni… …

…ng is given.

Mitigating the effect of a locked layout: plain iteration of instance… …

…s now is possible also with a locked layout. Before, the instances were correctly iterated only if the layout could be sorted.

update documentation