This is fulfilled automatically in regular FV methods, but in network situations
this may be violated.

The result looks better than previously -- not sure whether it is correct, though.

For the network case, this way to compute the flux should be 'more correct'
than the old one.  Not sure if the new version is correct yet, though.

The program runs, but the output doesn't look quite correct yet.

Refine it once

This is the initial commit of an example program solving the transport
equation using a finite volume method.  This first commit simply copies
an existing program using YaspGrid into the correct location.  Future
patches will replace the YaspGrid by a FoamGrid, and make it work
on an embedded network grid.