Xtrigger sequence fix

[hjoliver]

These changes partially address #3283 (companion PR to master will complete).

Major:

• record xtrigger functions against cycling sequence, and only use those valid for the sequence when task proxies are instantiated.

Minor:

• better unifies the handling of clock- and general-xtriggers
• wall_clock works with positional or keyword offset arg (another problem identified by @trwhitcomb)
• disallows clock triggers with integer cycling

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 (functional).
• Appropriate change log entry included.
• No documentation update required (this just makes xtriggers work as they should).

[matthewrmshin]

Generally happy with these changes. A few comments on style.

[hjoliver]

(rebased)

[dwsutherland]

LGTM 👍

Ran it on my system, and the xtriggers were on sequence.

(just Travis now)

[hjoliver]

Thanks @dwsutherland 👍

[hjoliver]

Yay, the 7.8.x test battery passed, unbelievable 😮 (I stole a few fixes from @matthewrmshin's work on master).