converts DrakeSystem/findFixedPoint over to the new NonlinearOptimizatio...

...n interface.  have not moved RigidBodyManipulator/findFixedPoint over yet, but that will come soon
In this case, what is `varargin`? If we give a cell of constraints depending on different subsets of decision variables, should `varargin` be a cell array, with each cell containing a cell array of `xind`? We should formalize the input argument of `addConstraint`, since this method is going to be extensively used. Also add documentation for this method