String serialization of collapsed coordinate.

This PR extends the capability of `Coord.collapsed` to allow it to serialize the collapsed points (and bounds) of a coordinate as a concatenated string for only those coordinates with `units` of `no_unit` or `unknown`.

This permits CF label (string) coordinates to be collapsed, when given the appropriate `units` value.

Note that:
 * This may result in a `dtype` change of the collapsed coordinate i.e. from numeric to string.
 * This serialization preserves all the `points` and `bounds` data post-collapse.