change derived_tasks argument to derived_task_ids

[zkamvar]

Not sure if this has been brought up before, but before we release this version of hubEnsembles, I believe we should change derived_tasks to be derived_task_ids to match the parameter in our schemas and other packages.

This also gets at the issue of the fluid representation of the word "task" in our terminology (e.g. in the preceding paragraph and this example we have task ID variable == task ID == task ID col, which are all ways to describe individual elements of the compound_taskid_set and derived_tasks).

Originally posted by @zkamvar in #154 (comment)