Get rid of the `rank` template parameter whenever possible and useful

[kordejong]

Pros:

• mdspan makes it easy to dynamically (at runtime) configure an array's rank (and shape)
• It may speed up the build. By itself and also because we need fewer explicit template instantiations.
• Increasingly, standard algorithms can handle spans (see linalg)
• ...

Cons:

• All code handling arrays, for example, need to test at runtime whether the rank is as expected.
• ...