Removed the special 'sequential' task type.
Reimplemented sequential tasks with inter-cycle triggers.
(Re)Documented sequential tasks.
Fixed some test suites.
Fixed a typo.
Minor coding efficiency improvements (credit M.Shin).
Corrected new obsoletion version numbers.
Corrected ready-to-spawn state list.
Sensible validation error for sequential async tasks.
Merge pull request #826 from hjoliver/810-remove-sequential-tasks

Removed the special 'sequential' task type.