Implement the Tradeoff model for delayed choice tasks

[drbenvincent]

• [x] implement the Tradeoff model
• [ ] add a notebook to examine the properties of the model

References

Scholten, M., & Read, D. (2010). The psychology of intertemporal tradeoffs. Psychological Review, 117(3), 925–944. http://doi.org/10.1037/a0019619