ENH: new multiFieldValue function object

    Computes a selected operation between multiple \c fieldValue function
    objects.

    The operation is applied to all results of each \c fieldValue object.

Note
    Each object must generate the same number and type of results.

Usage
    Minimal example by using \c system/controlDict.functions:

    multiFieldValue1
    {
        // Mandatory entries (unmodifiable)
        type            multiFieldValue;
        libs            (fieldFunctionObjects);

        // Mandatory entries (runtime modifiable)
        operation       subtract;

        // List of fieldValue function objects as dictionaries
        functions
        {
            region1
            {
                ...
            }
            region2
            {
                ...
            }

            ...

            regionN
            {
                ...
            }
        }

        // Optional (inherited) entries
        ...
    }

    where the entries mean:

      Property  ...