Use Enumerable#sum in ElectricityDemandCurve

* sum is slightly faster.

* Moved the implementation to `ElectricityDemandCurve#[]`, since this
  if more commonly called than `get`.

app/models/qernel/plugins/fever/electricity_demand_curve.rb

@@ -19,15 +19,15 @@ def length
       end
 
       def get(frame)
-        @producers.sum { |prod| prod.source_at(frame) }
+        self[frame]
       end
 
       def first
         get(0)
       end
 
       def [](frame)
-        get(frame)
+        @producers.sum(0.0) { |prod| prod.source_at(frame) }
       end
 
       # For some reason, Merit calls curve#values#[]