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 system/controlDict functions:

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

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

    // Optional (inherited) entries
    ...
}

Serves as a precursor to a more general function object based on the fieldExpression toolchain