use ControlPersist ssh option to fix ControlPath shutdown

fixes #448

Also adds docstrings.
Fixes #422

(pending) Create test_switchdpidassignment.py

IVSSwitch: turn off verbose logging by default

Most users don't need this much logging and it slows down the switch.

use ControlPersist ssh option to create a ControlMaster connection that will not die when a node dies

fix sudoers file to allow -u option

Note that we do still provide a default if you specify
ControlPath=True

Adding support for multiple custom files

Works in a few ways:
 * a single file: --custom foo.py (as before)
 * comma-separated list: --custom foo.py,bar.py
 * multiple custom args: --custom foo.py --custom bar.py

fix plot command to work when standard classes are present