Add method to ``sample'' particle positions.

I need to use it for many species of particles, so I added direct
control over the particle species name.

this is a significant change of the behavior of joint_rspace_PDF,
because I didn't realize I was doing something stupid...

Code now computes vorticity stats first, and then velocity stats.
This is relied upon in the `NSVEparticles` class later to avoid an FFT.

We should use conventions used previously in the literature, so that's
what we're doing.