PrivateCloud.java:
- Now it uses the external tool!

OptEngine.java:
- The calls to the PrivateCloud class should be fixed.

SolutionMulti.java:
- Now there's a method for reading all the available providers in a
solution file.
- I've combined the optimize() and optimizeMultiprovider() because it
was terrible to have two methods completely identical.
- Now it exports the solution even if it isn't evaluated.
- The total cost is a double now.

EvaluationServer.java:
- Now it also considers the private cloud instances.

CostEvaluator.java:
- If the solution is on a private cloud, it computes the cost
accordingly.