ENH: multiFieldValue - extended to operate on general function objects

- Previously, the multiFieldValue function object was limited to operate on
  lists of fieldValue function objects.
- Any function objects that generate results can now be used, e.g.

pressureAverage
{
    type    multiFieldValue;
    libs    (fieldFunctionObjects);

    operation   average;

    functions
    {
        inlet
        {
            type            surfaceFieldValue;
            operation       areaAverage;
            regionType      patch;
            name            inlet;
            fields          (p);

            writeFields     no;
            writeToFile     no;
            log             no;
            resultFields    (areaAverage(inlet,p));
        }
        outlet
        {
            type            surfaceFieldValue;
            operation       areaAverage;
            regionType      patch;
            name            outlet;
            fields          (p);

            writeFields     no;
           ...