Resolve #1 "Port Interface", #8 "Port Coupling"

Adds:

• Port interface functions for InputPort and OutputPort.
• Functionality for connecting ports and signals, both via signals and ports.
• Tests that verifies that the functionality has been correctly implemented.
• Fixes to older code so that it works with the new interfaces.
• Minor functionality for connecting operations in their constructors.

Closes #1 (closed) and closes #8 (closed)

test/graph_id/conftest.py

-import pytest