Updates cost interface, documentation

Static & dynamic cost were only separated because the original project
spec wanted us to complete static routing as an intermediate step
towards dynamic routing (which we skipped).  I've combined them into a
single cost method, and updated some of the related documentation.  Cost
is now calculated as propagation delay * data packet size * (1 + buffer
fill) / log(available capacity).  This is a dynamic cost, but is
equivalent what would have been static cost when traffic & buffer fill
are zero.