Fix sequence bounds check for parent tasks.

[hjoliver]

This change addresses a bug (and untested functionality, evidently) found by @TomekTrzeciak in #3740 (comment)

In the following example, bar should trigger off of foo in the first 3 cycles, then subsequent bar's should trigger automatically (no parents):

R3//P1 = "foo => bar"
P1 = "bar"

On this branch it works, but on master the scheduler shuts down finished after cycle point 3.

Since spawn-on-demand we auto-spawn parent-less tasks when their previous instance leaves the runahead pool (they have no parents to spawn them "on demand"). In this example, bar.3 has a parent (foo.3) but bar.4 has no parents and so has to be auto-spawned. To do this, when bar.3 leaves the runahead pool we look at the next cycle point for bar and see if it has parents there by looking for parents in each of its sequences. This parent-lookup gave the wrong result because sequence.is_on_sequence(point) disregards sequence bounds. The fix is to use sequence.is_valid(point) (which correctly says that bar.4 has no parents).

Marking as Draft until I add a test.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).

• Appropriate tests are included (unit and/or functional).

• No change log entry needed (fixes a very recent never-released bug)

• No documentation update required

• No dependency changes.

[hjoliver]

Proof the fix works:

$ cylc cat-log foo | grep triggered
2020-08-06T15:59:47+12:00 INFO - [foo.1] -triggered off []
2020-08-06T15:59:48+12:00 INFO - [foo.2] -triggered off []
2020-08-06T16:00:01+12:00 INFO - [bar.1] -triggered off ['foo.1']
2020-08-06T16:00:01+12:00 INFO - [bar.2] -triggered off ['foo.2']
2020-08-06T16:00:02+12:00 INFO - [bar.3] -triggered off []
2020-08-06T16:00:02+12:00 INFO - [bar.4] -triggered off []
2020-08-06T16:00:03+12:00 INFO - [bar.5] -triggered off []
2020-08-06T16:00:15+12:00 INFO - [bar.6] -triggered off []
2020-08-06T16:00:16+12:00 INFO - [bar.7] -triggered off []
...

[hjoliver]

(rebased)

[hjoliver]

Minimal functional test added (fails on master, passes here).

[oliver-sanders]

Good, tested with the following:

[scheduling]
    initial cycle point = 1 
    final cycle point = 9 
    cycling mode = integer
    [[graph]]
        R3//P1 = a => b
        P1 = b 
        6/P1 = b => c