Fix failure to get initial partition with EAGLE_100 volume.

Running the EAGLE_100 volume results in a failure to create an initial partition using METIS and a vertex only weighted graph with particle counts as the weights. Seen in !667 (merged).

This fixes that problem, which is that the total number of particles exceeds the value IDX_MAX (in this case INT_MAX). Also now includes counts for gravity and star particles and instead of counts we use the number of bytes per particle type, so we should get a partition that is more memory balanced (which was the initial intention).