Add Status-Related Data Models & DB Commands

In order for the status commands to start using the Merlin database, we need to store the necessary information in this database. We'll likely need the following data models:

• StepModel
  • name
  • description
  • batch settings (implement later when we refactor the batch.py file)
  • parameters -> this might need its own data model
  • associated queue
  • link to logical worker (if we do this, we don't really need to store the queue in the bullet above)
  • list of tasks associated with this step (link to TaskModel)
  • link to StudyModel
  • status? Not sure if this is necessary as we can build the progress bar from TaskModel statuses
• TaskModel
  • workspace
  • status
  • return code
  • time since last start
  • number of restarts
  • physical worker(s) that have processed this task
  • Note: if we save this for each task then I'm worried about how much space we'll take up in the database

We'll also need the ability to interact with these models via:

• merlin database get step <step id or name>
• merlin database get all-steps [--filters]
• merlin database delete step <step id or name>
• merlin database delete all-steps [--filters]
• merlin database get task <task id or workspace>
• merlin database get all-tasks [--filters]
• merlin database delete task <task id or workspace>
• merlin database delete all-tasks [--filters]

Start by locating the existing Merlin database models and the CLI entry point for merlin database. Read how current models and database commands are structured before defining the StepModel and TaskModel scope. Done means the required status data can be stored and the listed get/delete commands work with IDs, names, workspaces, and filters.