Resolve TMSS-200

Closes TMSS-200

See merge request !167

TMSS-207:  renamed TaskConnector to TaskConnectorType, and the related names of the input_role and output_role. Simplified and made code more generic for linking the created TaskBlueprints from the existing TaskDraft and TaskRelationDraft

TMSS-207:  it might look like producer has an incorrect related_name='consumed_by'. But it really is correct, denends on the way you look at it

TMSS-207: return the normal list of urls, but then sorted by url so they are easier to find in a list for us humble humans

TMSS-207: generalized serialization of reverse relations, so they are serialized in _ids lists as well (next to the normal hyperlinks)

TMSS-207: Update testcases for subtask unittesting. Renamed t_specify_observation to t_task. There were old call used which not existed any more.

TMSS-200: Remove SubtaskInputSelectionTemplate since and use TaskRelationSelectionTemplate instead. Fix test.

TMSS-200: Remove SubtaskInputSelectionTemplate since and use TaskRelationSelectionTemplate instead. Fix test.

Intermediate merge of TMSS-207 to master

See merge request !162

Resolve TMSS-175

Closes TMSS-175

See merge request !160

TMSS-207: added various convenience meta methods and REST actions to create and/or schedule the subtasks from the TaskBlueprint, TaskDraft, SchedulingUnitBlueprint or SchedulingUnitDraft

TMSS-207: added various convenience meta methods and REST actions to create and/or schedule the subtasks from the TaskBlueprint, TaskDraft, SchedulingUnitBlueprint or SchedulingUnitDraft

TMSS-207: raise SubtaskSchedulingException when start/stop times are None. add default start/stop times.