Iterators/generators on spreadsheet items

I haven't found any "lightweight" versions of the `get_*` methods, i.e. methods returning iterators or generators on the results. This makes reading a large spreadsheet memory-intensive. Is there any specific reason behind this choice?

No files or tests are named. Start by locating the spreadsheet get_* methods and their current result-loading path; determine which methods would need lazy iterator or generator variants and define completion around memory-efficient results for large spreadsheets.

Written by the indexing model from the issue text.