Changes required to replicate results in my thesis except color

This is a subset of the pull request discussed in https://github.com/ioam/topographica/pull/584. Everything related to color has been left out. Therefore, there are three changes left:
1) Extra buffer for patterns in case of motion simulations
2) Switch for input patterns (Gaussian/Images)
3) Gain control for spatial frequency
Ok, makes sense now; 'SF' is a property of the sheet, not the simulation.