As per #70 (closed), we will run into problems with more than 2bn particles per node as all the particle counters and loop variables are currently int.
As a first step, switch the stored particle counts to size_t.
The next series of commits will be to change all the loops over the particles. I'm breaking this into two pieces since these changes are highly disruptive, especially since I'm submitting them along with !113 (merged).