config: support offsets in hold and stop after cycle points

[oliver-sanders]

The final cycle point configuration supports offsets e.g:

final cycle point = 2000
final cycle point = 2000 + P1Y  # i.e. 2001
final cycle point = +P1Y  # i.e. "initial cycle point" + P1Y

However, the stop and hold cycle points do not.

It would be nice if these configs could work in the same way.

Context / why is this an 8.3.0 issue?

A common pattern in some areas is to run a workflow until it stops. Then modify the final cycle point to extend the workflow and restart.

This isn't an easy working pattern to port to Cylc 8 due to the difficulty of re-populating the start tasks. We have a plan to make this easier, see #5416, however, this working pattern is needlessly complex in the first place.

Swap final cycle point for stop after cycle point and the workflow never "completes" so the pool never needs re-polulating. This is much nicer for Cylc (no discontinuity), the user (no extra commands) and provenance (direct continuation of previous run).