Add a new type to CxxUtils for arrays of optional values

This merge request adds a type to the CxxUtils package, the OptionalArray, which for a given template type T wraps the std::array<std::optional<T>> type. It adds several useful helper methods which make the type more ergonomic to use. This can be used later in applications such as the Trk::TrackSummary, or other places where an array of optional values is required.

I've done my best to add comments and tests for maintainability.

This merge request doesn't change any existing code, and should thus not break anything.