Make the trait sealed by default

[Fokko]

Fixes #501

I'd like to suggest making the trait sealed by default. A sealed trait requires to have all the classes that it extends in the same file. Which is the case with scalaxb. Having a sealed trait makes it easier to spot errors because the compiler will drop warnings.

[eed3si9n]

Maybe this should be opt-in. I think there are situations where scalaxb would produce datatypes crossing multiple files when there are multiple schemas involved, and also I'd love to move code generation that's more modular, similar to what was done in https://github.com/sbt/contraband.

[Fokko]

Good point, I'll make it configurable, somewhere this weekend probably.

[dalegaspi]

i am revisiting this from #501 and the code change can't be like this, because this assumes that all generated traits are sealed...however, any generated traits that employ inheritance will not compile (as shown in the failed test) because (some?) of the traits are generated in different source file which violates the sealed property (sealed traits and its inherited objects/traits need to be in the same source file).

I am haven't looked at this in a while so I have no clue as to how to make this work...maybe add an option to configure the types to have a root trait as sealed and ensure that they are all generated in the same source file?

[deenar]

@dalegaspi @Fokko I was looking into this too as I was some of the typelevel Cats type classes and run into the same issue. I would too prefer if making all generated traits sealed was an opt-in.