Add `split_ports_sequentially()`, left-edge based `split_on_ports()`, and always default to the left-edge heuristic

Ping @oscgu95, the documentation is now failing due to new assignments as a result of left-edge being the new default heuristic. One should probably see that the comments make sense throughout the examples now, beyond just fixing the errors in them.

From a 'code running successfully' point of view, it seems as if only L. 134 in secondorderdirectformiir_architecture.py is failing. But again, the manual assignment in the examples might all be messed up now.

approved this merge request

merged