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)
They are used in different places for different things - unfortunately, they are used
so much (in the GUI) that it is much faster to cache and send the string along with the integer
unix time.