Defer loading pivot caches

Created originally on Bitbucket by jbquenot (Jean-Baptiste Quenot)

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

This change allows openpyxl to load the pivot caches only if needed, thus saving a tremendous amount of time when loading files with big pivot tables in unused sheets. This is related to issue #1038 (closed): Ability to load specific sheets, and only makes sense if specific sheets are loaded.