Add FPM build system

[yoricklassmann]

The Fortran Package Manager is an open-source project developed by the Fortran-lang community, and it makes building and distributing Fortran code so much easier and, in my opinion, nicer.

In case no one has strong objections to adding yet another build system (@danielhollas 😉), I can take the lead on adding this feature.

[danielhollas]

I am hesitant to support three build systems (we recently merged cmake support), but I'd be open to switch from make to fpm if it turns out to be simpler. I haven't used fpm yet, but from my understanding it has some requirements on how the project / source code is structured so I am not sure how much work in would be to make it work.

In particular, it would be great if fpm solved the problem of dependencies between the files. In src we have a perl script that autogenerates the deps for the top level files, but the dependencies between modules in modules/Makefile are currently maintained manually.

I would maybe hold off for now, since there is currently an ongoing work on interface with eT, and I'd like to see how that one works before considering this change.

WDYT Yorick?

[yoricklassmann]

I completely agree that supporting three build systems is probably (more than) a bit overkill. So I'll wait for now, but I still wanted to raise the issue, as I myself have found FPM to be very convenient, especially because it is just so simple.