Change CYLC environment variable to look at.
Merge pull request #1297 from arjclark/cal_to_cyling_rename

rose date: change CYLC environment variable to look at.
rose task-env now providing ROSE_CYCLING_MODE. rose date now using that.
Tweak documentation.
Merge pull request #1299 from arjclark/rose_task_env_link_cycling_mode

rose task-env/rose-date: use ROSE_CYCLING_MODE