Don't unnecessarily rewrite DataFrames for checks

By using df[column] = df[column].apply(check...) we were re-writing
the DataFrame every time we returned from a check. We don't actuall
y need to return a value at all, as the point of checks is to print
a warning to the screen. In Python a "return" statement without a v
ariable returns None.

I haven't measured the impact of this, but I assume it will mean we
are faster and use less memory.

csv_metadata_quality/experimental.py

@@ -94,4 +94,4 @@ def correct_language(row):
         )
 
     else:
-        return language
+        return