cylc play --re-run

[oliver-sanders]

Better to attempt after #3897

Add a --re-run option to cylc play to enable the workflow to be rerun without being reinstalled.

See the rose suite-run migration proposal.

• The user would first have to invoke cylc clean to move the log dir to log.<timestamp> (or just move / delete it themselves).
• Then cylc play --re-run can be invoked.
• If cylc play --re-run is invoked and the log dir exists the command should fail.

The proposal suggests that cylc play should --re-run automatically if the log directory is not present, however, the explicit presence of the --re-run argument is probably better for clarity / safety. Adding the question label (ping @dpmatthews).

Questions:

• --re-run automatically? - No

[dpmatthews]

The proposal gave 2 options but I think I prefer the explicit presence of the --re-run argument.

[oliver-sanders]

Ditto, will remove the question label.

[oliver-sanders]

This has now become absorbed into point (3) in #3887