Feat/workflow wrapper extensions

[seanpearsonuk]

An incremental improvement to the workflow interface:

• provide information about the task hierarchy
  -- which tasks are directly up/downstream of a given task, which is implied by task inputs/outputs. The methods are get_direct_upstream_tasks(), get_direct_downstream_tasks()
  -- get all ordered children of either the workflow or of a given task. By recursion, the full hierarchy can by obtained. The method is ordered_children() (also inactive_ordered_children())
• remove the class nesting in workflow.py because of documentation difficulties
• add test for the new functionality. Also update existing tests, reflecting a recent API change in the backend, which is effective in 2023R2

N.b. I have deliberately avoided changing rst files to avoid conflicts with @hpohekar.

[hpohekar]

@seanpearsonuk

Following change might resolve one of the issue related to api doc generation.

raise_method_not_yet_implemented_exception() -> _raise_method_not_yet_implemented_exception()

because for raise_method_not_yet_implemented_exception() also, the sphinx was generating a separate rst file.