Prefer single quotes and added source details

Whenever possible, the long names lists in nl_NL providers are now single-quoted strings like `'Amsterdam'`. Unless there is a single-quote in the name; like `'s-Gravenhage` (The Hague) which is then double-quoted to `"'s-Gravenhage"` (spot the single-quote inside the double-quotes...). Furthermore, this PR adds comments to data sets and implementations when a reference for them is known (as per #125 draft Coding Guidelines).