[323] Joint rupture builder experiment

[voj]

Satisfies #323

An experiment to create joint ruptures.

Includes

• DownDipFaultSubSectionCluster a subduction-specific cluster
• RuptureLoader loads ruptures from a rupture set and re-create classes such as DownDipFaultSection and DownDipFaultSubSectionCluster
• ManipulatedClusterRupture lets us merge existing ruptures in different ways
• JointRuptureBuilder can create sequential joint ruptures
• JointRuptureBuilderParallel can create parallel joint ruptures using splays
• JointRupturePostProcessor is the main class that drives the JointRuptureBuilder implementations
• FishboneGenerator is an experimental fishbone graph generator for joint ruptures

[voj]

• put in separate experimental namespace
• write warning log that this is experimental?
• readme in namespace
• go find other experimental stuff that's already merged