Update the MergedVolumeActor example (with LFS)

[JulienBert]

Update the MergedVolumeActor example:

• physics list opt3
• replace i33 file to mhd file
• remove the obsolete reference simulation
• merge Mac file into one main file, which is more readable
• change the name from "hybrid volume navigator" to "MergedVolumeActor"
• add a readme with few informations

[tbaudier]

Thank you Julien, lfs is ok I merged