CONFIG: add additional modules/ directory and Module namespace

- any code placed here should provide Allwmake and Allwclean scripts
  and normally have compilation targets into FOAM_APPBIN, FOAM_LIBBIN

  Since there is no standardize places for sources or applications,
  a simultaneous build of a module's doxygen documentation requires
  a minor bit of manual effort. Add (via symlink) the sources into
  the modules/doc/ directory to have them included in the normal
  OpenFOAM doxygen documentation generation.

  A makelink.example file is provided there as an example.