Stop writing test weather models into repo

[royagrace]

Tests now symlink the input weather files into a scratch tmp directory instead of tracked repo directories, so derived output lands outside the repo.

Description

Followed example from PR #814

Made changes based on PR Review notes:

Deleted the two tracked derived timeInterp .nc files, moved test_slant.py/test_datelist.py output and temp.yaml under tmp_path so those tests stop writing into the repo, and removed the blanket gunw_azimuth_test_data/ .gitignore entry so future accidental writes there show up in git status instead of being silently hidden.

Worth flagging in the PR: anyone with locally-modified copies of the two removed .nc files may see them as changed when pulling this — that's expected, not a conflict.

Motivation and Context

How Has This Been Tested?

Ran autopep8
Ran local tests

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.