New issue 847: Total mass fails with zero particles
After loading a Flash dataset (cosmological simulation) with gas and DM particles I tried
to take the total mass within a subvolume but because there were no DM particles within
the region it failed. The call should return zero mass, or at least some error handling
should stop it from breaking.
Note: reg.quantities.total_mass() works if reg = ds.h.all_data() but does not if reg =
ds.sphere("max", (.2, "kpc")) for example.
Traceback can be found at http://paste.yt-project.org/show/4699/