Remove default ‘top’ assembly from Project, etc

Remove default ‘top’ assembly from Project
Clean up passthoughs server API, move tree-building logic to gui client
  . passthroughs frame gets updated when assembly is modified
  . passthroughs frame remembers expanded/closed state between updates
Enable drag and drop from dataflow to workflow in gui
Increase background grid size in gui diagrams
Update gui tests and clean up some gui test code
Remove passthrough trait if passthrough creation fails
Remove unnecessary ‘is_instance(self, Assembly)’  test in assembly get_attributes
Some linting and code commenting