[Feature] Tidy up wing disc scripts

[alessandrofelder]

Is your feature request related to a problem? Please describe.
The wingdisc functionality is almost ready for an initial template generation run 🎉
The current PR #60 has been useful for experimentation, learning and feedback.

But is currently a bit big and the code could do with tidying up. There are some files that don't belong in the codebase (mirror_test.py and argument assignments).

Describe the solution you'd like
We add each of the points below as separate PRs, in this order:

• add the loader functions and the [wingdisc] extra dependency, open a new PR
• add a cleaned example script in a new PR. Cleaning should include
  - [ ] using np.flip instead of our mirror function
  - [ ] removing any commented-out code
  - [ ] add code to write the brain_paths.txt and mask_paths.txt files we need

We can worry about tests after.