add additional information in header of decomposedBlockData

We already have a blocks entry, but it would be quite convenient for downstream users (eg, paraview) if we also could pass along the underlying data-type in an easy to find place.

Eg,

FoamFile
{
    version     2.0;
    format      binary;
    class       decomposedBlockData;
    blocks      4;
    object      U;
    dataClass   volVectorField;
}

At the moment it is a bit of a pain to get at this information.

Would it be useful to have this dataClass information present as IOobject::headerDataClassName() with it being identical to IOobject::headerClassName() when dataClass isn't present?