In galaxy (or mpte, or any other machine, including your laptop) we
want to make test particle simulations. We might make multiple simulations
with just 1 particle in each, or 1 simulation with multiple particles. In the
context of our present work, we might run *several* simulations, each with a
fixed intial mu. Alternatively, we might run 1 simulation with many values of
mu. Sometimes we might also want to *combine* the results from several
simulations, such as when we are evaluating the diffusion coefficients.