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)
Ah, I think that is one of my "TODO ISO" comments that is no longer relevant. At the time I was thinking "bounded range" tasks just disappear from the suite once their final cycle point is done, so is there any reason not to do the same with tasks spawned beyond the suite final cycle point.  But then I decided suite final cycle point is different to the end of a bounded recurrence: in principle at least we might want to be able to change or cancel a shutdown before it has occurred, but couldn't do that if tasks beyond the original stop point had disappeared.  So, the current situation is exactly what the comment says to "check that we're not", but is the correct behaviour if you agree with my reasoning there.