Improvement/flexible task iteration

[essweine]

This PR includes

• a rewrite of task iteration to make it more extensible: it was moved out of the Task class and better filtering capabilities were added
• duplicative workflow methods that returned specific types of tasks were removed and Workflow.get_tasks now takes other keyword arguments besides the state and filters them all at once, in one pass through the tree
• duplicative task methods were removed
• some previously private methods are now public
• translations between task state names and values is now handled by the TaskState class, which was moved into the SpiffWorkflow.utils.task module.
• documented public methods in Task and Workflow

[essweine]

Also, both methods (get_tasks and get_tasks_iterator) already existed.

The advantage of task filter is that you can extend it if you have custom properties and use your own instead (so for example, you could write a filter in arena's backend that includes spiff properties and pass that in). I added this because there have been many times when I've been hampered by the builtin iterator's capabilities.