Shared/Date::isDateTime Handle Cells Which Calculate as Arrays

[oleibman]

In issue #4557, the user complains, with some justification, about the way Excel handles certain calculations. We are not able to help with that problem. However, the user also notes a problem in Shared/Date when isDateTime has to evaluate a cell whose calculated value is an array. This is solved by flattening the calculated result to a single value.

It became obvious while working on this change that the code to set instanceArrayReturnType was kind of awkward. Simpler methods returnArrayAsArray and returnArrayAsValue are added to Spreadsheet. Even these started out a bit awkward because Spreadsheet::calculationEngine was defined as nullable, which really isn't true. It is allocated by the constructor, and never freed except in the destructor. It is no longer nullable.

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