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
The ```dataset``` parameter would need to be changed to be a param.Parameter. Then, ```if self.dataset=='Gaussian'``` needs to be changed to something like ```if(isinstance(self.dataset,imagen.patterngenerator))```. Finally, the ```image_folder``` variable could be deleted and ```self.dataset``` directly passed to ```PatternCoordinatorImages```.
It shouldn't be too much work, but I don't think I get to change it before my vacation.