[WIP]feat: worker debugging

[philipp-seitz]

initial draft to mock worker functions by adding a default value that is returned instead of invoking the task worker.task(default_value=1).

The user can overwrite this behaviour manually:

debug_values = {iadd: lambda x, y: x + y + 1, itimes: _my_itimes, isubtract: 5}

This will only be used in a new function debug_graph that uses the InMemoryStorage and InMemoryExecutor to run the entire graph within one process.