013. New Fortran code should use free source form and .f90 suffix#

topic: Style

New Fortran code should be written using free source form and stored in a file with a .f90 file extension. Free form was introduced in the Fortran 90 standard, but using the .f90 suffix does NOT mean the code is restricted to Fortran 90 features. Don’t use the .f95, .f08, .f18 suffixes, which are not recognized by many tools.