More consistent use of combineReduce, simpler and/or reductions

- replaced ad hoc handling of formatOptions with coordSetWriter and
  surfaceWriter helpers.

  Accompanying this change, it is now possible to specify "default"
  settings to be inherited, format-specific settings and have a
  similar layering with surface-specific overrides.

- snappyHexMesh now conforms to setFormats

  Eg,

      formatOptions
      {
          default
          {
              verbose     true;
              format      binary;
          }
          vtk
          {
              precision   10;
          }
     }

     surfaces
     {
         surf1
         {
             ...

             formatOptions
             {
                 ensight
                 {
                     scale   1000;
                 }
             }
         }
     }