Add build targets for source code formatting

This makes it possible to run:

• make format-fortran to format Fortran source files with fprettify;
• make format-c to format C/C++ source files with clang-format;
• make format that does both.

Although .clang-format is there, most (if not all) of the C/C++ source files in the repo are not formatted according to it. I don't want to mess with the authorship of the lines in git blame, so I leave it to the core developers to decide what to do about it.

If this is accepted, it should replace the existing logic in the CI, I guess.