Avoid dot notation for CZML properties `number`, `boolean`, and `string`

While not officially [JavaScript reservered words](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Reserved_Words); some tools can't handle JavaScript properties named `boolean`, `number`, or `string`, which we use in CZML.  Changing them from dot notation to string based lookups works around these issues and has no affect on performance.