Making Cactus Installable

Making Cactus installable like any other Ubuntu package has several obvious benefits:

• HPC centres can install Cactus, ensuring it is available by default in many places
• Beginners have an easy way to get started, which may be even simpler than our current download instructions
• Cactus would not seem "strange" and "different" any more; the notion would shift from "putting things _into_ Cactus" to "using Cactus", which is what it should be