Add MPI Usage Example to the Project

[MatthAlex]

The current Fortran project template does not include an example showcasing the usage of MPI (Message Passing Interface) for parallel programming. Adding an MPI example will showcase how to migrate a project to this template.

Some ideas:

• Create a new example directory under examples/ for MPI usage.
• Develop a simple Fortran program that utilizes MPI for parallel processing.
• Update the fpm.toml file to include the necessary MPI dependencies.
• Document the MPI example in the README.md, including instructions on how to compile and run the example.

[MatthAlex]

This is a headscratcher, mostly because of fpm limitations.

FPM

The main work to solve this issue is shown in this PR@fpm. Until it's fixed, fpm will try to build everything with MPI if the dependencies require it.

MPI project

In this repository I've outlined a simple MPI project using this template.

The main change that enables this is fpm.toml:

$ diff fortran-project-template/fpm.toml ../mpi-project-exploration/fpm.toml 
< implicit-typing = false
< implicit-external = false
---
> implicit-typing = true
> implicit-external = true
> [dependencies]
> mpi = "*"