This is required for the bindings, because the 2D views on 4D tensors
must be on the correct type (i.e. pytensor/rtensor/xtensor). Although,
casting of the views to become xtensor could have been used instead.

In doing so, there is no source file to be compiled anymore, so this
becomes a header-only library. But to keep the "public" headers separate
from the implementation, a subdirectory "detail/" is added.