include/vsg/traversals headers

The include/vsg/traversals header directory contains the main traversal classes, currently includes the beginnings of cull and dispatch traversals but as the project progress these will be joined by other traversals such update, event and other traversals that are found to be useful.

• include/vsg/traversals/CullTraversal.h - Currently just a shell. During Prototype Phase will be implementing basic cull traversal that will cull nodes with bounding volumes against view frustum, accumulate state, and create a dispatch graph to passed on to the dispatch traversal to pass data and commands to Vulkan.
• include/vsg/traversals/DispatchTraversal.h - Very early cut of dispatch traversal that dispatches data and commands into to a Vulkan command queue. Currently only tracks a subset of Vulkan state components and does so with relatively slow std::map<> containers. This will be fleshed out further in Prototype Phase to enable basic scene graph rendering capabilities.