Add a way to validate a FrameRepr

[thvdveld]

The values of a FrameRepr can be anything right now. We try to prevent this by providing a FrameBuilder. However, the fields of FrameRepr are public, meaning an invalid representation can still be constructed. I don't like to make these fields private, since then we need to implement getters for the fields and this would make the API clumsy.

We could however add a validate function to FrameRepr that returns an error when an invalid frame is constructed.