235 include example data

[BalzaniEdoardo]

In this PR we address issue #235 and we move osf_download out of tests.

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[BalzaniEdoardo]

@billbrod remember to remove unused figures in src/plenoptic/data.

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Codecov Report

Attention: Patch coverage is 82.50000% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 86.70%. Comparing base (8971b43) to head (0d98e23). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/plenoptic/tools/external.py	25.00%	6 Missing :warning:
src/plenoptic/data/fetch.py	86.20%	4 Missing :warning:
src/plenoptic/tools/data.py	76.92%	3 Missing :warning:
src/plenoptic/data/__init__.py	93.75%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #252 +/- ## ========================================== - Coverage 86.73% 86.70% -0.04% ========================================== Files 35 38 +3 Lines 3325 3392 +67 ========================================== + Hits 2884 2941 +57 - Misses 441 451 +10 ```

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[billbrod]

I've expanded upon Edoardo's changes:

• removed data/ directory at the root of the repo: we can just download those images (used for tests) from the OSF
• removes my custom functions for downloading data from osf, now uses pooch to do so, which has variety of useful functionality
• standardized how notebooks get images (some were still calling plt.imread, which will fail)
• if torchvision not installed, we no longer install it in notebook, but raise an error telling people to install it.
• removed unecessary images from src/plenoptic/data
• restructured how user interacts with plenoptic.data, to look more like scipy's: separate method for each image they can load in.