Houdini: Point at center of primitive VEX

I didn't come up with this, but it's so handy I'm just gonna put it here (credit to this odforce post)

This this in a per-primitive wrangle to get a point at the center of each prim.

// Adds a point at the position of each prim
addpoint(0, @P);
// Remove primitive and all points connected to it
removeprim(0, @primnum, 1);