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     | Description                         | Type | Req'd | Dflt
      type         | Type name: multiFieldValue          | word |  yes  | -
      libs         | Library name: fieldFunctionObjects  | word |  yes  | -
      operation    | Operation type to apply to values   | word |  yes  | -
      functions    | List of fieldValue function objects | dict |  yes  | -
    \endtable

    Options for the \c operation entry:

       add           | add
       subtract      | subtract
       min           | minimum
       max           | maximum
       average       | average

Deprecated fieldValueDelta

- The fieldValueDelta function object was originally written to compute the
difference between two fieldValue-type function objects. The multiFieldValue
object name better describes its purpose whilst being able to operate on an
arbitrary number of fieldValue-type objects.