Don't unnecessarily import from modules depending on pygraphviz.

Closes #525.
Fixed polling of orphaned tasks on restart.

The restart command had not been updated for very recent polling
developments; and task poll timers have to be created before job
submission so they can still be used for orphaned tasks resurrected
on suite restart. Closes #524.
Some comments.
Documented how remote control works.
Merge pull request #527 from hjoliver/525.graphviz-dependence-revisited

Don't unnecessarily import from modules depending on pygraphviz.
Handle late 'submission succeeded' messages properly.

The started message can arrive first if a task starts running very quickly.
Merge pull request #528 from hjoliver/524.fix-polling-on-restart

Fixed polling of orphaned tasks on restart.
Add blurb on monitoring other suites.
Merge pull request #529 from hjoliver/doc.remote-control-issues

Documented how remote control works.
Merge pull request #530 from trwhitcomb/monitordoc

Add blurb on monitoring other suites.
Final cug update for 5.3.0.
Fix graph command for namespace graphing.

The recent --ungroup option only applies to dependency graphs.
Merge pull request #531 from hjoliver/fix-cylc-graph-namespaces

Fix graph command for namespace graphing.