size_t

As per #70, 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.

See merge request !114