maintain Fluid.Examples.DrumBoiler

  - add conditional external inputs q_F and Y_Valve for optional use as sub-model
  - add animation of fluid level to EquilibriumDrumBoiler component


git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6122 7ce873d0-865f-4ce7-a662-4bb36ea78beb