New synthetic stimuli module.

[colah]

New module for generating synthetic image stimuli. It presently only supports a (very general) rounded curves stimulus, but provides a more general framework.

This has two major change from previous iterations (found in colab notebooks):

• Creating a "sampling" function which gives one a great deal of flexibility as to how a stimulus is rendered. I think this is a natural division of responsibility, and gives end users a lot of control over their stimulus family.

• Switching from a traditional curve stimulus to a more flexible "rounded corner" (based on the rmin function).

[colah]

(I listed Ludwig and both Nicks as reviewers to give them an FYI, only need a review from one person.)

[colah]

... I think it's possible that these are now so slow to render that rendering them is the bottleneck on collecting activations. :(