Convert gcylc stopped state to str in case it is None.
Suite state now documents and validates valid states.
Minor tidying up.
A reporting change and a fix for repeated config items.

* reporting: repeated graph sections don't warrant a warning
* bug fix: don't just warn about repeated item override, do it
Correct output for 'requested state'.
Merge pull request #630 from hjoliver/repeated-config-sections

A reporting change and a fix for repeated config items.
Merge pull request #629 from arjclark/cylc-suite-state-docs-validate

cylc suite-state: validate inputs and improve documentation
Fix casting of datetime in scheduler.
Merge pull request #631 from arjclark/fix-shutdown-datetime

cylc shutdown|stop: fix using a datetime for shutdown
Merge pull request #626 from hjoliver/gcylc.str-stopped-state

Convert gcylc stopped state to str in case it is None.