title: Metric (total-cost) not minimized with Metric-FF
body: <p>I'm trying to use Metric-FF (v2.1) to solve a classical planning problem. I noticed that my plans weren't optimal and started to play around with cost minimization. I realized that the planner doesn't seem to take the cost into consideration despite the cost-minimization being activated. 
  I've created the following <strong>minimal example</strong>, with one predicate and two identical methods (besides their cost):</p>
  
  <pre><code>(define (domain metrictest)
      (:requirements :strips :action-costs)
      (:predicates (done ?x))
  
      (:functions (total-cost) - number)
  
      (:action do1
          :parameters (?x)
          :precondition (not (done ?x))
          :effect (and (done ?x) (increase (total-cost) 1))
      )
  
      (:action do2
          :parameters (?x)
          :precondition (not (done ?x))
          :effect (and (done ?x) (increase (total-cost) 5))
      )
  )
  </code></pre>
  
  <p>And all I want to solve is: </p>
  
  <pre><code>(define (problem problem_1)
      (:domain metrictest)
      (:objects foo)
      (:init (= (total-cost) 0))
      (:goal (forall (?t) (done ?t)))
      (:metric minimize (total-cost))
  )
  </code></pre>
  
  <p>Which results in <strong>do2</strong> to be called despite <strong>do1</strong> being the cheaper operation. If I change their position in the domain file, do1 is picked.</p>
  
  <p>If you know any obtainable planner solving this correctly, I'd be happy use it.</p>