Now, to reference mininet files, use 'import mininet.mininet'.

PYTHONPATH mods are no longer required for installation.

Unfortunately, I can't test this at the moment since nox seems not to
be able to find the routing module.

Added/renamed examples: grid.py, tree1024.py

Added -v flag to nox arguments, so we have some chance of figuring
out why nox is dying, as it generally is.