Added rows and columns to nav_init()

It doesn't make sense to init a nav array without rows and columns
so include it in the init function so we don't forget to init them.
Having trouble with the relays

We seem to be getting bad values off the IR sensors when an object
goes out of range. It isn't a problem on any of the other sensors
which leads me to believe that it is a problem with the relays.
I will investigate.