Implementations of is_contiguous and make_contiguous

[SanggyuChong]

In this PR, we attempt to roll out python-based functions that allow one to check whether or not the values in a TensorBlock and TensorMap are contiguous, and make them contiguous as needed. Tests for the newly implemented operations have also been implemented.

Resolves #265

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

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📚 Documentation preview 📚: https://metatensor--522.org.readthedocs.build/en/522/

[github-actions]

Here is a pre-built version of the code in this pull request: wheels.zip, you can install it locally by unzipping wheels.zip and using pip to install the file matching your system

[jwa7]

Looking good so far @SanggyuChong , nice job!

I've requested a few changes below. You'll notice on the PR that the build failed - this is because of a linting error. You can use tox -e lint and tox -e format to run the linter and formatter respectively to make sure changes to the code adhere to the style guides in metatensor.

After you've made these changes, let's discuss how to make a metatensor-torch version of this function - probably best done in person.

[jwa7]

Also, if you look at the documentation build here: https://metatensor--522.org.readthedocs.build/en/522/ and navigate to the operations you'll see that these operations aren't yet there. In general 'documentation' in the checklist at the top of this PR covers all of comments, docstrings, API docs, and tutorials - where necessary and appropriate. In this case we'll need an API reference but will have to manually do that - we can discuss this too in the next step!

I am now taking over the PR