This is to make way for a new `Tuple` class that mimics `std::tuple`,
but can be reliably used on device and with aligned Eigen types.
The existing Tuple has very few references, and is actually an
analogue of `std::pair`.
This provide several advantages:
- more flexibility in designing unit tests
- unit tests can be glued to speed up compilation
- unit tests are compiled with same predefined macros, which is a requirement for zapcc
