Implement unary wedge

[Erik Schnetter]

It is sometimes convenient to call a multiplication operator with a single argument, as in \wedge(xs...) where xs is a collection of chains. For example, * returns its argument when called with a single argument. It would be convenient if \wedge did the same; this would avoid code such as

            if length(xs) == 1
                vol = abs(xs[1])
            else
                vol = abs(∧(xs...))
            end