ENH: Function objects - added new 'multiply' function object

Multiplies a given list of (at least two or more) fields and outputs the
result into a new field.

    fieldResult = field1 * field2 * ... * fieldN

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

    multiply1
    {
        // Mandatory entries (unmodifiable)
        type    multiply;
        libs    (fieldFunctionObjects);

        // Mandatory (inherited) entry (runtime modifiable)
        fields  (<field1> <field2> ... <fieldN>);

        ...
    }