merge "119.iso8601 cycling" to master, for cylc-6

:guitar: for those about to rock ... :bomb: :boom:  **cylc-6**

addresses #119 
 
(there are still some details to attend to before hitting the *merge* button ... but let the review begin)
I'm not sure that it should - the `add_to_runahead_pool` call makes it tricky.
I'm a fan of this kind of thing being in the task pool.