Implement fixed direction stepping for multiple time steps

[TomTranter]

When a walker hits a wall it is bounced back to where it came from then continues randomly under the assumption of Brownian motion. It would be possible to fix the trajectory after hitting the wall for a certain number of steps to increase the effects of wall interactions as opposed to particle-particle interactions. This could be done either by setting a fixed number of steps or adjusting the probability of moving in a particular direction along a particular axis. The second way is probably better from a physics perspective but might be trickier to implement as it would require storing movement probabilities for each walker.