Use training/tuning/testing dataset terminology

**Have you checked the [list of proposed rules](https://github.com/Benjamin-Lee/deep-rules/issues?q=is%3Aissue+is%3Aopen+label%3Arule) to see if the rule has already been proposed?**

- [x] Yes

**Did you add yourself as a [contributor](https://github.com/Benjamin-Lee/deep-rules/blob/master/contributors.md) by making a pull request if this is your first contribution?**

- [x] Yes, I added myself or am already a contributor

**Feel free to elaborate, rant, and/or ramble.**

I would suggest the use of training/tuning/testing rather than training/validating/testing or (God forbid) training/testing/validating to avoid any confusion in different domains. I see inconsistent use of this terminology in existing issues and draft text. See #19 for an example.

**Any citations for the rule?** (peer-reviewed literature preferred but not required)
- https://twitter.com/michaelhoffman/status/989251677646704641
- https://twitter.com/michaelhoffman/status/989972609042395136
- https://twitter.com/CarldeBoerPhD/status/989318174431764480