RFC: Clear internal model if problem modification won't catch it

This clears the internal model if the user adds something that won't get caught in the problem modification process:

* linear constraints
* quadratic constraints
* SOS constraints
* quadratic objective
* callbacks

We may want to eventually support these types of modification (assuming the solvers support them), but until then we should try to ensure correctness.

There's also a small fix in the model.jl test independent of these changes, so that should me merged into master regardless.
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