CZML versioning

**NOTE** #1944 should be merged before this gets looked at.

We now throw a `RuntimeError` under the following conditions:

1. The first CZML packet in a load operation is not a `document` packet.
2. The first `document` packet does not contain a `version` property.
3. The `version` property on any document packet is not `"1.0"`.

This goes along with AnalyticalGraphicsInc/czml-writer/pull/76
Might be a good idea to use `defaultValue` to use the defaults for `ClockRange` (and `ClockStep`) if the string is unrecognized.