Relax assumption on BaseFormOperator's dual argument slot

[nbouziani]

This PR relaxes the assumption on the type of the object that goes into the dual argument slot of BaseFormOperators. Base form operators (e.g. ExternalOperator or Interpolate) of the form B: V -> V, or B: V x V* -> R, have an argument slot to represent their dual argument (the one in V*). Strictly speaking, whatever is in V* should be allowed to go in there. At the moment, the code assumes that only a Cofunction or a Form can be held in this slot. This PR extends that assumption to other cases such as Action or even BaseFormOperator.