fixed some comments.

no really, nothing to see here, I swear!
added a move method that only takes a single segment.

turned the multi segment method into a helper function. this is
more in line with the current plan for how we'll actually use this
stuff.
now the platform motion code will tell the robot to stop when there's no
path

this is probably a good thing, but might actually conflict with other
control modes. I'm going to have to think about that...
keep the new pvt code from stomping over all other kinds of command
sources

also keep it from listening to the planner when it isn't supposed to.
that seems like it could be bad.