Improvement/better subworkflow management

[essweine]

This PR splits adds a simple BpmnSubworkflow class that

• keeps track of the task id that controls the subprocess
• allows better navigation inside nested subprocesses
• adds a depth property

The order of execution inside do_engine_steps has changed. Tasks are completed by subprocess, starting with those at the lowest depth. This allows do_engine_steps to refresh waiting tasks whenever all the available tasks in a subprocess are complete. This should make it more intuitive to use.

It also cleans up some methods in both Workflow and BpmnWorkflow, and removes some BPMN related attributes that have migrated into the base Workflow class.