functionObjects::subtract: From the first field subtract the remaining fields in the list

    The operation can be applied to any volume or surface fields generating a
    volume or surface scalar field.

    Example of function object specification:
    \verbatim
    Tdiff
    {
        type            subtract;
        libs            ("libfieldFunctionObjects.so");
        fields          (T Tmean);
        result          Tdiff;
        executeControl  writeTime;
        writeControl    writeTime;
    }
    \endverbatim