Read Anndata objects in backed mode (from disk) wherever possible

[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)

[pavanvidem]

everytime a dfferent linting issue :)
Tests pass.

[bgruening]

.. ERROR: Error 'HTTPSConnectionPool(host='hastie.su.domains', port=443): Max retries exceeded with url: /ElemStatLearn/ (Caused by ConnectTimeoutError(<HTTPSConnection(host='hastie.su.domains', port=443) at 0x7fa1ba5f5e90>, 'Connection to hastie.su.domains timed out. (connect timeout=5)'))' accessing https://web.stanford.edu/~hastie/ElemStatLearn/