feat(fortran): Fortran interface can now use multi-zone

Fortran interface uses the new C api ability to call the naieve
multi-zone solver. This allows fortran calling code to make use of in
build parellaism for solving multiple zones

tests/extern/fortran/meson.build

@@ -1,5 +1,11 @@
-executable('test_fortran_extern', 'gridfire_evolve.f90',
+executable('gf_fortran_single_zone_test', 'gridfire_evolve_single.f90',
   install: false,
   fortran_args: ['-Wall', '-Wextra'],
   dependencies: [gridfire_fortran_dep]
-)
+)
+
+executable('gf_fortran_multi_zone_test', 'gridfire_evolve_multi.f90',
+           install: false,
+           fortran_args: ['-Wall', '-Wextra'],
+           dependencies: [gridfire_fortran_dep]
+)