Converting derived classes

[ecorm]

Add provisions for converting to/from derived classes. Boost.Serialization has a boost::serialization::base_object cast operation, so we might need something similar as well.

Also add an example of converting derived classes in the tutorial.

[ecorm]

It turns out that Boost.Serialization needs to handle base classes in a special way because of object tracking. The object tracking feature does not apply to CppWAMP, so there's no need to handle base classes in a special way.

There should still be a test cast and an example covering the conversion of derived classes.