Adds Pytest teardown to close figures

[billbrod]

Describe the change in this PR at a high-level

This PR adds pytest fixtures so we can use them as a teardown to run after tests that create plots. This way we can close tests correctly (no matter how the test ends) and automatically (so we don't have to remember to add it to each test).

Also figured this out for doctests, where the proper thing to do is to add a conftest.py to the same directory tree as the doctests, i.e., the src/ folder. (See docs).

I have added the teardown:

• to all doctests (with autouse)
• to all tests in tests/test_display (with autouse)
• to a select few tests in tests/test_models (without autouse, needs to be individually activated)

I could've just put it in test/conftest.py and automatically applied it to all tests, but the overwhelming majority of our tests don't create matplotlib figures, so that seems like overkill.

Checklist

Affirm that you have done the following:

• I have described the changes in this PR, following the template above.
• I have added any necessary tests.
• I have added any necessary documentation. This includes docstrings, updates to existing files found in docs/, or (for large changes) adding new files to the docs/ folder.
• If a public new class or function was added: I have double-checked that it is present in the API docs, adding it to one of the rst files in docs/api/ or adding a new file as necessary.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/plenoptic/conftest.py	0.00%	6 Missing ⚠️

Files with missing lines	Coverage Δ
src/plenoptic/conftest.py	0.00% <0.00%> (ø)

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