[WIP] MNT: Use an image fetcher class instead of global variables.

[hmaarrfk]

Description

@rfezzani this is the idea of using a class I had to avoid the generous use of globals out of order.

Honestly, at this stage, it doesn't fix any of the issues that are brought up, but I feel like it is a better starting point.

Feel free to take it over.

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

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

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

Line 119:80: E501 line too long (80 > 79 characters)
Line 124:80: E501 line too long (83 > 79 characters)
Line 128:80: E501 line too long (81 > 79 characters)
Line 146:80: E501 line too long (81 > 79 characters)
Line 201:80: E501 line too long (82 > 79 characters)
Line 227:80: E501 line too long (80 > 79 characters)
Line 243:1: E305 expected 2 blank lines after class or function definition, found 1

[rfezzani]

That's even better @hmaarrfk 👏. I would also through this class definition out from the __init__.py file to keep it as short/clean as possible ;)
I'll try to find some time to work on this ASAP =)