functionObjects::add: Add list of fields

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

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

Also refactored functionObjects::fieldsExpression to avoid code
duplication between the 'add' and 'subtract' functionObjects.