Add Flatten layers support

@txir-met pointed out these are a really super common thing to have and we haven't got them. To add them we would need to do the following three tasks:

- write a Fortran subroutine representing them;
- write an Ennuf Model representation that can write out appropriate calls to that subroutine;
- write translations from supported libraries' representations of this layer to Ennuf Model representations of it

No files or tests are named. Start by locating existing layer implementations, Fortran subroutines, Ennuf Model representations, and translations from supported libraries; done means all three Flatten-layer tasks are implemented and the relevant existing tests or examples pass.

Written by the indexing model from the issue text.