ENH: add -root option for wmakeLnInclude (convenience)

- finds the correct root directory location before creating
  the lnInclude directory

  Eg,

  from within something like src/finiteVolume/fields/fvPatchFields/..

      wmakeLnInclude -update -root

  it backtracks to find the top-level directory with Make/
  and makes the lnInclude directory there:

    Using /home/mark/openfoam/OpenFOAM-plus/src/finiteVolume
        ln: /home/mark/openfoam/OpenFOAM-plus/src/finiteVolume/lnInclude