Disabled GIZMO particle steering for vacuum particles

I disabled particle steering for particles with zero density, as they have weird sound speeds that mess up the steering algorithm. This fixes #271 (closed).

mentioned in issue #271 (closed)

Thanks. @jwillis could you test this one as well with ICC ?

It works fine with ICC up to ~1300 steps and is still running. How many steps did you run it to with GCC?

I ran 733 steps on my computer. But last week I managed to run until the end (time 0.01), and then it took 4880 steps.

No crash report from @jwillis so I presume it's ok to merge ?

I think I ran it up to ~2000 steps in the end and it was fine. So yeah it's okay to merge.

Thanks !

mentioned in commit b620c08f

mentioned in commit 1b2c8ee9

Status changed to merged