ENH: make "file" property in functionObjects a relative path (issue #1125)

- partial solution for issue #1091

  This generates file properties that are case-relative,
  Eg,

      plane0
      {
          p
          {
              file "<case>/postProcessing/plane0/1/p_plane0.vtk";
          }
          U
          {
              file "<case>/postProcessing/plane0/1/U_plane0.vtk";
          }
      }

   This allows the case to be moved elsewhere and still find its files.

   This functionality was previously added for vtkCloud, but now also
   applies to streamLine, sampledSets and sampledSurfaces