Fix: Set default surface name for compatibility with Cantera 3.1

The ck2yaml that comes with Cantera 3.1 needs
the surface site to be named. Because there's no 3.2
binaries in conda-forge for Python 3.9, we'll end up
installing 3.1 in most people's environments, so
let's be compatible with it (even if it's wrong).

Author: rwest

rmgpy/chemkin.pyx

@@ -2168,7 +2168,7 @@ def save_chemkin_surface_file(path, species, reactions, verbose=True, check_for_
     sorted_species = sorted(species, key=lambda species: species.index)
 
     # Species section
-    surface_name = None
+    surface_name = 'SURF0' # Some ck2yaml versions (Cantera 3.1) require a name.
     if surface_name:
         f.write('SITE/{}/'.format(surface_name))
     else: