Headers content and include directory

[GiudGiud]

There's two main things that I believe we should change in how we use headers

• the function definitions, docstrings, should go in the headers rather than the source .cpp files. That way whoever is looking at how to use a function does not have to look at its implementation.

• the headers should be all together in an include directory. This would make it easier to find a source file or a header file, and also hides implementation for whoever is just looking to call OpenMOC from another code.

I'm posting this issue to see if there's any opposition or interest in doing this.