WIP:C++ refactor struture

This is not meant to be merged

I thought I would check in and show the direction I am going in for the start of refactoring, so I could get some feedback before I go too deep doing the wrong thing.

I have added a new CXX source file with the start of the refactored interface, which is included in libtng.so. So far I have started to break up data structures and functions into (largely blank) classes and methods.

I have aimed at a deeper C++ interface than the current C++ wrapper (tng_io.hpp) which covers all the methods, but is super lightweight, making almost no changes to the actual data structures.

Any feedback appreciated, I'm more looking for design ideas or anything i'm doing which I shouldn't be.