Allow manual triggering of retrying tasks.
Merge branch 'master' into manual-trigger-retrying-tasks

Conflicts:
	lib/cylc/task_types/task.py
Merge branch 'master' into manual-trigger-retrying-tasks

Conflicts:
	lib/cylc/scheduler.py
Merge pull request #475 from hjoliver/manual-trigger-retrying-tasks

Allow manual triggering of retrying tasks.
Merge remote branch 'upstream/master' into cylc-graph-ungroup-opt