getInputChannelName() and getOutputChannelName() (pure virtual in the
superclass) aren't returning anything. So far this hasn't triggered
any memory errors because nobody's calling them.

To hack around this, have both of these return
"xxx-UNUSED-OPEN-EPHYS-xxx" (none of the existing subclases override
these methods, so leaving them pure virtual isn't workable). This way,
if we ever start calling these, we'll notice right away that a fix is
needed (instead of segfaulting, etc.).

Also keep them virtual to allow overrides.

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>