ENH: E&Hu halofit class and documentation

[jucordero]

Description

Added EisensteinHuHalofit class and its corresponding documentation. Still missing unit tests Class includes the model attribute which selects between three parameter sets for halofit: Bird, Smith and Takahashi.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [x] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [ ] Assign someone from the infrastructure team to review this pull request