Provide additional model classes that are currently missing (e.g. annotations, panel links, series overrides, etc.)

[bascarsija]

Note that #11 can provide a means of ensuring that such missing model entities can still be preserved during roundtrip modification.

Please note that I have created a fork that already implements this, and can produce a PR if this functionality is desired in the main code base.

[pgiles]

Please create this PR and I promote to the main code base