used to look up hyperedge info given a vertex. Essentially,
we store the hypergraph as a sparse matrix in both CSR and CSC formats.
This doubles the memory cost but gives better performance.
The data on each processor is stored using local indexing, starting at zero.
In order to get the global vertex or edge number, use the macros 
<i>VTX_LNO_TO_GNO</i> and <i>EDGE_LNO_TO_GNO</i>. These macros will 
look up the correct offsets (using the dist_x and dist_y arrays). 
Note that <i>phg->nVtx</i> is always the l