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)
This was just a decision of mine to only allow the basic ISO 8601 format e.g.
```
initial cycle point = 20110101T0000
```
rather than the extended:
```
initial cycle point = 2011-01-01T00:00
```
in suite.rc (and CLI) date/times, as the first is what would actually be output.
I've since changed my mind, as they're all valid ISO 8601.