updating naming conventions of different API

[kminhta]

This PR:

• updates the naming convention of the different API options throughout the repo and docs
  • aggregator-based workflow -> Task Runner API
  • director-based workflow -> Interactive API
  • workflow interface -> Workflow API

TODO:

• [x] Update workflow_interface notebook names to workflow_api and requirements_workflow_interface.txt to requirements_workflow_api.txt
  • [x] within notebooks, need to update the requirements cell to point at updated name