Refactor mlpack header to be similar to ensmallen.

[shrit]

Hi,

Just a placeholder for this task so we do not forget about it, no priority on it for now.

Since mlpack is a header only library it would be nice to have all the binding and the tests in one place and all the methods and core are under include/
This will require some refactoring on cmake level as well.

[rcurtin]

That would make the codebase more in line with typical C++ projects that split headers into include/ and .cpp files into src/. I think it's a good idea, and also provides more clear logical separation between bindings/tests and the mlpack library itself.