Add support for fixing "unnecessary" Unicode

These are things like non-breaking spaces, "replacement" characters,
etc that add nothing to the metadata and often cause errors during
parsing or displaying in a UI.

csv_metadata_quality/app.py

@@ -25,6 +25,9 @@ def main(argv):
         # Fix: whitespace
         df[column] = df[column].apply(fix.whitespace)
 
+        # Fix: unnecessary Unicode
+        df[column] = df[column].apply(fix.unnecessary_unicode)
+
         # Check: invalid multi-value separator
         df[column] = df[column].apply(check.separators)