Neatly indent coordinates

[twpayne]

Currently <coordinates> are emitted on a single line, e.g.

<coordinates>1,2,3 4,5,6 7,8,9</coordinates>

It would be nice if coordinates, when emitted with WriteIndent were neatly indented, e.g.

<coordinates>
  1,2,3
  4,5,6
  7,8,9
</coordinates>

This would required Coordinates.MarshalXML to "know" whether it is being called in pretty or compact node. This information is currently not exported by the standard encoding/xml library.

[twpayne]

This makes for nice human-readable documents, but KML is usually consumed by machines, so closing this for now. Please re-open if needed.