TMSS-1218: Add parsets to SIPs and refer to the primary observation subtask as ObservationID (former 'SAS VIC Tree Id')

TMSS-778: Add error_reason field to Subtask model; update filterset and serializer for the Subtask model

TMSS-159: further adaptations to new start/stop times, add aggregate time properties, set process times when scheduling

TMSS-917: ensure that each parent task_blueprint has exactly one primary child subtask; added and adapted tests

TMSS-917: now that we reverted to a single parent TaskBlueprint for a Subtask there is no need anymore for a SubtaskOutput to have a task_blueprint reference.

TMSS-917: refactored the parallel LBA calibrator and target observations into one task with one specification with two parts (for the calibrator and the target). This enables us to have a single parent task for each subtask again, which is preferrable

TMSS-917: added readonly primary property to subtask, and made the subtask creation methods fill in the correct value for primary/non-primary subtasks

TMSS-745: fixed inheritance for Meta class in various models. Otherwise base Meta properties (like constraints) are missed.

Updated changes

TMSS-761: added readonly property SAPs which returns a QuerySet containing all SAPs of the subtask's output dataproducts

Bug Fixed

TMSS-703: Cancel running obs on triggered obs, add project property to subtask, modify path_to_project to deal with ManyToMany fields

TMSS-725: processed review comment: split the ERROR state into (generic)ERROR and UNSCHEDULABLE which are both end-states.

TMSS-725: fixed tests, and added convenience method to set a subtask to a desired state following the allowed transitions.

TMSS-725: added a SubtaskAllowedStateTransitions table, populate it with the allowed state transitions, and raise an exception if a user tries to submit an illegal state transition