Add two datasets for use in upcoming scientific tutorials.

[mkcor]

Description

• data.protein_transport() shall be used in Add textbook-like tutorial on measuring fluorescence at nuclear envelope. #5262
• data.nickel_solidification() shall be used in a tutorial in preparation with @cgusb

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @mkcor! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage/data/_registry.py:

Line 163:80: E501 line too long (101 > 79 characters)
Line 164:80: E501 line too long (98 > 79 characters)
Line 178:80: E501 line too long (107 > 79 characters)
Line 179:80: E501 line too long (109 > 79 characters)

Comment last updated at 2021-12-04 15:27:20 UTC

[grlee77]

not too big of a deal, but it looks like these were mostly alphabetical previously? (although text was out of place).

[mkcor]

Oh, sorry, that's because I first named it 'solidification' 😅 ('lily' was also out of place). Reordering the entire list... (except for 'data_dir', 'download_all')

[hmaarrfk]

out of curiosity, how does this help scientific tutorials? Won't you need to cut a release in time?

[mkcor]

@hmaarrfk the dataset with cell images is for use in #5262 (which I will update as soon as I'm in front of my PC, i.e., probably tomorrow).

[hmaarrfk]

Ah, understood.

[grlee77]

@meeseeksdev backport to v0.19.x