bfgsmin iterations

[Przemek Klosowski]

   // this is cache on feval to prevent the extra evaluation of the
   // objective function when subsequent calls with exactly the same arguments would take place

I think we have to be cautious with modifying feval to cache the results, because this
makes two assumptions:

- the function is 'functional' i.e. should return the same result given the same argument
  This is not true e.g. for stochastical functions (monte carlo, etc).

- the function has no side effects, i.e. doesn't change some global state.