Improve `sized` flag logic

[argonaut22]

@argonaut22 added a field to the aircraft struct to indicate if a model is sized. Right now it's v naive (namely, reaching the end of wsize).

We should figure out how to do this more robustly (e.g., not setting ac.sized = True if wsize() doesn't converge or if a driving parameter is changed [this may depend on determining "design parameters"])

[ngomezve]

This is a very good idea, @argonaut22. In my code, I artificially set some fields to NaN if the sizing fails, but an approach such as this is much better.