reload after xtrigger arg change

[hjoliver]

Reportedly, cylc vr on a running workflow (i.e., reload) does not update the clock trigger time. Stopping the scheduler first (i.e., restart) does get it right.

[UPDATE]
Confirmed, using a minimal example:

[scheduling]
    initial cycle point = now
    [[xtriggers]]
       # x = wall_clock(offset=P0Y)
        x = wall_clock(offset=P1Y)
    [[graph]]
        PT1M = "@x => foo"
[runtime]
    [[foo]]

[hjoliver]

I think this probably affects all xtriggers, not just clock-triggers.

Draft fix posted.