Moved call to topo_update to advanc and removed from b4step2 and filrecur routines.  This call is now before the parallel threads are launched to loop over grids so should be thread safe, and also means topo_update is only called once at this time rather than unnecessarily for every grid.  Still need to check if there are other places we need to update topo during regridding, for example.
Merge pull request #62 from rjleveque/new_dtopo

Moved call to topo_update to advanc