[C++-sig] virtual functions and bind size reduction

[Ralf W. Grosse-Kunstleve]

> The second module is linked against the first one, so I think that the

> linker should load all shared libraries before any python::boost code
> get executed.

Python needs to execute the init function in the first module. I.e. you have
to have an import from Python.
You could do the Python import from the init of the second module, but
for quick testing, just insert the import somewhere so that it appears
before you expect the dynamic_cast to work.