It's possible that mn_clean is a bit easier to read; mnclean may be easier
to type, however.

Added variables to Makefile so that source files are listed in a single
location.

This required a change to logging, which now uses a singleton pattern.

For all future checkins, 'make codecheck' should pass.

Also remove usused imports and outdated doc text.

Having a file with the same name as its package creates hard-to-diagnose
import error, so rename mininet.py to net.py.  This commit moves utility
functions, generally those dealing with network namespaces and interface
management, into their own file.

Also fix a few minor formatting bugs and clean up logging code (which
had issues when multiple files would import mininet.logging_mod).

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

PYTHONPATH mods are no longer required for installation.

Added retry() function for createLink to see if it helps (probably won't.)
Random edits to docs.

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.

Network may now be used with custom controllers.
An example of doing this is in nox.py, which instantiates at TreeNet
using a custom Controller, NoxController, that runs nox_core rather
than the reference controller.

Fixed xterm.py (and cleanup) to clean up screen sessions.
Cleaned up sshd.py (though interface is still in flux.)
Added 1024-node network example (treenet1024.py).
Added example showing multiple tests on a single network (multitest.py).
Renamed examples to make them easier to type!

Other minor cleanup.

Modified cleanup to use dpctl to remove kernel datapaths.

Node -> { Host, Switch, Controller }
Network -> { TreeNet, GridNet -> LinearNet }

Modified cleanup to clean up kernel datapaths.