Changes required to replicate results in my thesis

This PR consists of several smaller changes, with the biggest change being the introduction of code for the new color model.

The changes are as follows:
1) When 'dr' is used, the position_bound is increased. This makes sure the pattern is swept across the connection fields, even for neurons at the edge.
2) A switch to use image datasets is introduced. Please note that the standard McGill images are not compatible. Instead, images with 3 channels have to be used with the new color model.
3) Introduction of code for the new color model.
4) Add possibility of gain control for the spatial frequency channels. I had some more tweaks for my thesis, however that was more playing around with different extents of the gain control projections etc.; however, there was no scientific reasoning for that so I left them out. Spatial frequency needs further work anyway.

Please note that some of the changes are a bit hackish (although I tried to make them the least hackish as I could). If you think they are too hackish and it's easier for you to change than for me, please feel free to make the required changes yourself outside this pull request.
I would favor saying num_cones or the like, rather than sim_type, which is very vague.  Right now it supports dichromatic and trichromatic, but there are humans with any number of different cone types (certainly 2 to 4, and possibly 1 (monochromatic), and arguably more (with multiple L and M types per person).  I'm not planning to model all these, but it seems reasonable to specify it by number rather than a vague "type".