Support multithreading for charge drift simulation

When simulating the drift of a charge cloud with

``` Julia
drift_charges!(evt, sim, Δt = time_step, max_nsteps = 50,diffusion = true, self_repulsion = true, verbose=true)

simulate!(evt, sim)

```

I don't see any improvement on the performance by previously setting

``` Bash
export JULIA_NUM_THREADS=10

```
to 10, 1 or any other number of threads. I do get a faster calculation in the case of the potential and weighting potential calculation, but not here. Is there any extra argument or step that I am missing?

Right now I am getting rates of 1 event/second for a highly segmented detector. As you can imagine, simulating 1E6 events becomes painfully slow...