Warning (instead of crash) for undefined species in load_transport_file

Previously load_chemkin_file crashes if there is an extra species in 
the transport file that is not in the model. Some transport files 
(not generated in RMG) that I've received from other sources will 
have some extra species in the transport file.
This change allows the user to "skip" redundant species in the transport
file in the load_transport_file function without crashing.

rmgpy/chemkin.pyx

@@ -896,10 +896,12 @@ def remove_comment_from_line(line):
     return line, comment
 
 
-def load_transport_file(path, species_dict):
+def load_transport_file(path, species_dict, skip_missing_species=False):
     """
     Load a Chemkin transport properties file located at `path` and store the
     properties on the species in `species_dict`.
+    If skip_missing_species=True then species not defined in the species_dict 
+    are just skipped over, with a warning.
     """
     with open(path, 'r') as f:
         for line0 in f:
@@ -909,6 +911,10 @@ def load_transport_file(path, species_dict):
                 # This line contains an entry, so parse it
                 label = line[0:16].strip()
                 data = line[16:].split()
+                if skip_missing_species:
+                    if label not in species_dict:
+                        logging.warning(f"Skipping transport data for unknown species {label}")
+                        continue
                 species = species_dict[label]
                 species.transport_data = TransportData(
                     shapeIndex=int(data[0]),
@@ -999,7 +1005,7 @@ def load_chemkin_file(path, dictionary_path=None, transport_path=None, read_comm
     # If the transport path is given, then read it to obtain the transport
     # properties
     if transport_path:
-        load_transport_file(transport_path, species_dict)
+        load_transport_file(transport_path, species_dict, skip_missing_species=True)
 
     if not use_chemkin_names:
         # Apply species aliases if known