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)
Can we have a consistent style for slotting strings into these errors - we have here three different types:
 * `"something %s" % my_var` (line140)
 * `"something " + my_var` (line 153)
 * `"something", my_var` (line 149)