Multiple voxel model homework

This exercise follows the same form as {doc}github_pca_homework and {doc}github_glm_homework.

As before:

Make sure you have logged into your Github account;
Go to the [course github organization] page. You should see a private repository with a name like yourname-multi-model-exercise where yourname is your first name.
Fork this repository to your own account;
Clone your forked repository;
Change directory into your new cloned repository:
```
cd yourname-multi-model-exercise
```

Make a new branch to work on, and checkout this new branch:

git branch multi-model-exercise
git checkout multi-model-exercise

Finish the dummy coding exercise by filling in multi_model_code.py;
When you've finished, commit multi_model_code.py, and push these changes to your github fork;
Make a pull request to the upstream repository.

Any problems, please feel free to contact JB or I.

Happy multi-modeling.