lastgenre: Add comments over groups of methods

trying to get a little order in the chaos. Maybe reordering and/or
moving out of the main plugin logic would be a better idea for some
methods but don't put much more refactoring into this PR to keep it
readable.

beetsplug/lastgenre/__init__.py

@@ -166,6 +166,8 @@ def sources(self):
         elif source == "artist":
             return ("artist",)
 
+    # More canonicalization and general helpers.
+
     def _get_depth(self, tag):
         """Find the depth of a tag in the genres tree."""
         depth = None
@@ -255,7 +257,7 @@ def _is_allowed(self, genre):
             return True
         return False
 
-    # Cached entity lookups.
+    # Cached last.fm entity lookups.
 
     def _last_lookup(self, entity, method, *args):
         """Get a genre based on the named entity using the callable `method`
@@ -303,6 +305,8 @@ def fetch_track_genre(self, obj):
             "track", LASTFM.get_track, obj.artist, obj.title
         )
 
+    # Main processing: _get_genre() and helpers.
+
     def _get_existing_genres(self, obj, separator):
         """Return a list of genres for this Item or Album."""
         if isinstance(obj, library.Item):
@@ -444,6 +448,8 @@ def _get_genre(self, obj):
 
         return None, None
 
+    # Beets plugin hooks and CLI.
+
     def commands(self):
         lastgenre_cmd = ui.Subcommand("lastgenre", help="fetch genres")
         lastgenre_cmd.parser.add_option(