Would be useful to have periodic boundary conditions working

[gregtucker]

It would be great to be able to handle periodic ("wrap-around") boundary conditions. There is an ancient and currently non-functional version of this in the set_looped_boundary_condition function and the corresponding "looped" node BC type. This node-BC type is not recognized by the set_status_at_link functionality, with the result that any attempt to access status_at_link after invoking set_looped_boundary_conditions causes an assertion failure.

This is probably an easy fix - maybe just a matter of adding the looped BC code to the function that identifies active links - but there may be some subtleties, and any fix will need some thoughtful test cases to go with it, to make sure it works both with continuum problems and flow routing.