Move all Fortran files to .F90
Created by: fxcoudert
This pull request is a possible fix for https://github.com/Slookeur/Atomes/issues/2
It makes all Fortran files .F90, including the ones that are incorrectly .f90 but have preprocessor directives. There is no downside to preprocessing all Fortran files, including the files that currently have no preprocessor directive. Therefore, I think it is the simplest fix.