Changes to INDEX Function

[oleibman]

Fix #64 (really!), closed as stale in December 2017, another in our "better late than never" series. Excel's INDEX function doesn't really behave quite as described. If a single row is used as an argument, either in literal form {item1, item2, item3} or expressed as a range A1:A6, INDEX is happy to evaluate the array as if each entry were a row rather than a single item. PhpSpreadsheet is changed to do likewise.

INDEX also returned #REF! when it would normally return an array (which would often be reduced to its leftmost topmost entry later). This code is deleted, invalidating one existing test, and INDEX will now operate like other functions which can return arrays.

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.