Related Issue(s):

• Closes Removing hierarchical links / add source link  #179
• Begins adding fixtures for Use pytest fixtures #948, and changes fixture names as recommended in Use pytest fixtures #948 (comment)

Description:

• Adds remove_hierarchical_links to STACObject
• Adds Links.is_hierarchical

I personally prefer the term "structural", but "hierarchical" was already in use so I stuck with that:

Includes the first pytest fixture for the test cases. I went for a more descriptive catalog name (label_catalog) instead of test_case_1 -- my thought is that it'll be nicer to depend on a well-named fixture rather than some arbitrary numbering scheme. I also followed @jsignell's recommendation and remove the test_ prefix from some existing fixtures.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.