Support calling functions on derived types

[adetaylor]

Follow on from #195. We need to make functions callable on derived C++ types.

[adetaylor]

#592 might be a good first step here.

[adetaylor]

The test case here should be duplicated to cover both virtual and non-virtual functions being callable on derived classes.

[badicsalex]

Hi,

I'm actually struggling with this one currently (while bringing g2o to the rust world), and I've made some warcrime-level workarounds to call "mut self" methods on an instance. I'm not really looking for actual casts (even though that would be nice), so even duplicating the methods on the derived type would be great.

I'd love to help in the thinking part of #592 too, that would pretty much solve this issue for me.