Shared Formulae outside the filter range when reading with a filter are not always being identified

[MarkBaker]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Issue #3473

When loading a file and accessing cells outside a filter range, we still check for shared formulae that may need to be used inside the filter range; so we test for the cell's f attribute; and if it is a shared formula then it's saved in the Shared Formula table so that it can be used when we encounter a cell inside the filter range that does use it.

However, in some cases (like this one) no f attribute was set and we need to check for the presence of an f child element with a shared attribute to indicate a Shared Formula that needs saving in the Shared Formula table.