Fix dynamic seeding following Thread::Pipe changes

The most recent change to multi-threading (issue #20, commit c8e39558) allows Pipe functors to return false to avoid sending data down the queue, rather than terminating the thread.
This stopped tckgen with dynamic seeding from terminating, as previously it depended on the file writing pipe class terminating in order to stop execution.
In this change, a streamline weight of 0.0 is used by the file writer class to indicate to the sink class (the seeding fixel map updater) to terminate, thus propagating thread termination back through the multi-threading queues.