BUG: fix missing @writer in write_fortran_namelist

Decorates io.espresso.write_fortran_namelist with @writer so can accept str filenames.

closes #1895 (closed)

Checklist: x (done) or ~ (irrelevant)

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Changes have been added in changelog.d using scriv.
• Issue is resolved via "closes #XXXX" if applicable.