Concat vertially and horizontally

Hey.
This is not exactly a PR... I think we need to discuss how to make this work.

  * how to put static methods on a class? 
      * we can export them as a function
      * we can not export them at all (as it is now)
      * we can change them to `concat_vertically_with` and make them methods.
      * other?
  * where to put our own C code?
      * separated in a `c` folder (as it is now)
      * blended in the swig `.i` files.
      * other?