ENH: add optional agglomeration coefficent to random decomposition

- specifies the number of consecutive cells to assign to the same
  randomly chosen processor. Can be used to have a less extremely
  random distribution for testing possible breaking points.

Eg,
    method random;

    coeffs
    {
        agglom  4;
    }

- Add finiteArea cellID (actually face ids) / faceLabel and procID
  for foamToVTK with -write-ids. Useful when this type of information
  is needed.