Spec out a 2D experiment space for testing

[rhyolight]

I think we should start simple with a small space like MNIST. So imagine something like 28*28 and a total of 784 input units, but topological.

Each location in this space:

• has a unique location representation
• may have a feature

We can put off specifying the feature type etc. until processing time.

If we do it this way, we can also use MNIST as a test set for object classification when we are ready, and we'll have the flexibility to decide what "features" are within each experiment.

[Zbysekz]

2D space is now defined; MNIST can be implemented in the future in #18