Slk Shared Formulas

[oleibman]

Fix #2267. The Slk format has a way to express a "shared formula", but the Slk reader does not yet understand it. Thanks to @SheetJSDev for documenting the problem and pointing the way towards a solution. It has taken a long time to get there. Part of the problem is that I have not been successful in getting Excel to use this type of construction when saving a Slk file. So I have resorted to saving a Slk file where shared formulas could be used, and then editing it by hand to actually use them. It would not surprise me in the least to have neglected one or more possible ways to specify a shared formula; but, at least the issue as documented is resolved, and if new issues arise, we'll probably be in better shape to deal with them.

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?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.

[oleibman]

No concern with Scrutinizer "complexity" message. Scrutinizer also flags one statement; as is often the case, this is a false positive, and will be filtered out from now on.