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I designed NavierStokes.prepare_launch without any particle addition
mechanism, since I only care about children of NavierStokes having
particles, but then I realized that for some very simple tests I should
have basic particle functionality.

particle file no longer gets generated if nparticles is 0. I'm not sure
I like the current solution, but it's better than nothing

also, set "allocate_time" to early for particle datasets. it does seem
to have some negligible effect...

this doesn't actually help

this way, it's easier to handle launching of children classes.