Python wrapping, compiler warning fix and enabling extensions on OSX again.

Hey Karl,

A few weeks ago I found your code from the midas-journal that enables multi-volume rendering in VTK. I took the liberty of adjusting the cmake build system in order to enable Python wrapping so that I can use it in my own project. I've also fixed a compiler warning and I've enabled loading extensions again on OSX so that I actually get some nice renders!

I'm not entirely sure whether I should send this pull request to your repository here, or to the midas-journal repository (or both). What do you think is the best thing to do?

I hope you like the changes!
Cheers,
Berend Klein Haneveld