I7 : FYPP files
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The use of `fypp` pre-processing is accepted, with limitations:
the preprocessing must happen within a Fortran `MODULE`, or within a `SUBROUTINE`.
In the latter case, it should not generate more than one actual Fortran subroutine/file from a fypp subroutine/file,
and the interface of the subroutine should not be changed by the pre-processing between the `.fypp` file
and the `.F90` file.

A bit of context
----------------

Since CY48, a new file type has been introduced in the IAL repository in order to reduce boilerplate code
and automatically generate Fortran code otherwise cumbersome to write and maintain.
This file type is called `fypp` for "Fortran pYthon Pre-Processing",
named after a tool that processes comments in Fortran code to expand it.
The files are suffixed `.fypp` and transformed into `.F90` files by the `fypp` pre-processor, prior to compilation.
Until CY49, only Fortran modules had to be pre-processed that way.
In CY49T1, 4 subroutines were transformed into `.fypp` in order to generate code within.

In the IAL, the use of `.h` interfaces files for subroutines within the project is enforced in order to detect bugs
at compilation stage rather than at execution.
However, in order to avoid writing and maintaining manually these interfaces in the repository,
they are automatically generated as a pre-compilation step.
This auto-generation only targets `.F90` files containing one subroutine, as stated in the coding standards of the IAL.

When it comes to `.fypp` subroutines, their interfaces need to be auto-generated too.
Two approaches are possible:
* transform `.fypp` into `.F90`, then generate interfaces
* generate interfaces of `.fypp`, then transform `.fypp` into `.F90`
For the second approach to be viable, it assumes that one `.fypp` produces one and only one `.F90` subroutine,
and that the interface (or signature) of the subroutine is not changed by the `fypp` pre-processing.