Updated the change log for 5.4.0
Update graph string interp doc.
Updated User Guide on inter-cycle triggers at start-up.
Updated User Guide on warm starts.
Graph view option to crop runahead tasks.
Check default runahead limit being enforced.
Add special task testing for sequential and one-off tasks.
Graph view: also crop tasks held beyond final cycle time.
Merge remote branch 'upstream/master' into expand-battery-8
Merge remote branch 'upstream/master' into expand-battery-8
Added testing for special tasks.
Tidy.
Merge branch 'master' into release-prep-5.4
Documented triggering off tasks in other suites (by polling).

Also updated the User Guide for the recent REGDB changes.
Updated the change log for 5.4.0.
gcylc back compat for new err log functionality.
More gcylc backward compatibility with 5.3.0 suites.
Merge branch 'master' into graph-view-runahead
Change runahead tasks to held if beyond the stop time.

Closes #751.
Upped some test timeout intervals.