Added convenience constructors to AllureModel.

I've added partial classes to the AllureModel to include constructors with the used parameters in order to use them directly instead of the language property initializers.
Also, I suspect the AllureModel.cs generated code was modified after generation which means that the modifications will have to be reapplied if the model will ever be generated again.

If the changes made to the model are only constructor implementations I suggest to move them too to partial classes and regenerate the model again to keep it clean in order to be able to regenerate it easily in the future.

I've added default constructors to keep any code that might use them from breaking and changed the implementation of the library to use the new constructors but that change may be discarded and just provide the added constructors as convenience for users of the library.