Scanpy read anndata in backed mode to reduce memory consumption

[pavanvidem]

FOR CONTRIBUTOR:

• I have read the CONTRIBUTING.md document and this tool is appropriate for the tools-iuc repo.
• License permits unrestricted use (educational + commercial)
• This PR adds a new tool or tool collection
• This PR updates an existing tool or tool collection
• This PR does something else (explain below)

There are two labels that allow to ignore specific (false positive) tool linter errors:

• skip-version-check: Use it if only a subset of the tools has been updated in a suite.
• skip-url-check: Use it if github CI sees 403 errors, but the URLs work.