Allow negative integer cycle points?

[hjoliver]

We fail this:

initial cycle point = -10

validation result:

$ cylc val flow.cylc 
IllegalValueError: (type=cycle point) [scheduling]initial cycle point = -10

But @kinow has discovered that the command line accepts negative points, and that the workflow then runs just fine:

$ cylc play --initial-cycle-point=-10 blah/run1

Job log path seems to be fine with - chars present too.

? should we

• officially accept negative cycle points because they work fine
  • and banning them seems arbitrary
• or ban them as an unnecessary complication
  • (the actual point values in integer cycles tend not to be meaningful ...are they ever?)

[hjoliver]

Meeting: allow it, make validation consistent.