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
Can't we avoid this hard-coding by making the name of the database be the same as the name of the image_folder?   And is there a good reason it's limited to Gaussians?  Could we e.g. accept either a string, in which case it's interpreted as the name of the image_folder, or else a PatternGenerator class (e.g. imagen.Gaussian)?   

If you think it would be better to merge this and *then* make such a fix, that's fine, but if so please write down a specific list of what needs to be done after the merge; I know there was already something else postponed for that.