Add support for cumulative viewshed

[abellgithub]

This adds support for cumulative viewsheds.

I've changed the C++ interface from earlier a little bit, but it's trivial (some support structures were moved out of the Viewshed class and placed in a namespace and all classes were put in that same namespace). I don't think it will change again.

There is a doxygen error but I think that's because I moved the code into a subdirectory. Maybe doxygen is confused because the documentation exists.