Enable final cycle specification as an offset.
Feedback changes 1.
Fix bad behaviour for integer cycling with P.
Merge remote branch 'upstream/119.iso8601-cycling' into relative_end_point

Conflicts:
	lib/cylc/config.py
Enable T06 style final cycles.
Try ... except handling used to handle integer cycling.
Merge pull request #1055 from arjclark/relative_end_point

#981: final cycle specification as an offset.